Why must restraints and pain be employed to elicit submission in a satisfying way for most Dom's?

Assuming you understand that there are people in our community you can do without…

Because restraints and pain are necessary to elicit submission in a satisfying way for most submissives.
I never asked my wife if she liked anal sex because my kind of woman prefers not to be given options. I also know that she would do anything to please me, and that is what pleases me the most. Had it repulsed her, I would have had to rethink what I wanted as anything that does not make her 'tremble with lust' would not please me either. She is an object of my pleasure, and it pleases me to pleasure her.
There is more to us than sex, but I would have to say that sex is the most important part of our relationship. But, it is not 'just sex'. It has never been 'just sex'.
Yes, I love women. For their mystery, their complexity and for the opposite. Adriana is her own woman to everyone except myself. She's been trained to follow my commands. She submits only to my will, my voice and my touch. In all else, she is equal in the world. The one definitive philosophy behind which I stand is that sub should be bent to the Dominant's will, but she shouldn’t be broken. Each sub requires a different touch.
Some of the things I do to her may superficially resemble abuse, but there's an important difference, I am doing these things because we both enjoy them, and in a manner that is respectful of her limits and desires. Treating ones partner with respect means treating your partner the way she wants to be treated - no law says that nice guys never fuck their wives in the ass or tell their girlfriends to strip and masturbate in front of them. It creates a deep intimacy that's hard to beat. This kind of exploration and sharing creates a bond of intimacy and trust that's more romantic and more passionate than you may believe.
I focus on my partner, and what she needs, social preconceptions of who "women are" mean nothing to me. My focus is on the woman I am with rather than what "women want."
I respond to taking charge in the bedroom, and she responds to being dominated. I enjoy being Dominant and I imagine it's much more fun to submit to a person who enjoys being dominant.

Pain as a necessary means to dominate?

Why would I want to deliberately hurt my most precious possession?
Ah, that's a bit tricky. I've been involved in BDSM and D/s long before I had words to describe either. The best single safety tool we have is our common sense. D/s is arguably one of the most complex forms of all human relationships, and it's different for everyone--something that works for one person doesn't apply to another. Mastering one set of protocols no more makes you an expert than mastering macaroni and cheese makes you a five-star chef.
The experience of pain in an erotic context, for someone who's wired that way, is nothing like what you may imagine; it's an incredible rush that adds a powerful spice to sexual pleasure. Think of it like spice in chili; you might not like taking a bite out of a hot pepper, but in the right amount, it makes the chili a whole lot better...
I've met many people who engage in BDSM activities, such as bondage or spanking, but who insist they are "not into that BDSM stuff." Usually, it's because they have an idea in their heads about what BDSM is, like "BDSM means wearing a leather mask and being chained to a wall and whipped, and I don't like that, so I'm not into BDSM." But BDSM doesn’t necessarily mean wearing a hood and being chained to the wall. If you like being lightly spanked or light bondage excites you, then you're into BDSM.

Some people, myself included, love the aesthetic of an elaborate form of bondage; others simply aren't interested in the bondage elements at all. Some people, myself included, love eroticism of caning, the pain, her submission and especially how lovely her bottom looks afterwards. The key to all these different forms of BDSM, though, is the exchange of power or sensation. One person is choosing to allow the other person to have control over him or her in some way, or to inflict sensation in some way.
Even things that look extreme, such as flogging or whipping, can be deceptive. These things don't feel like you imagine they do, and in the right environment with the right warm up, they're wonderful. For the most part, it's more stimulating than painful. Once her endorphins, the natural painkilling chemicals produced in the brain, get going, it's the most intense, delightful high you can ever believe.

Pain itself is a very complex topic. It has been researched for decades and while we understand fully how pain receptors and chemicals involved work, we still do not quite understand how pain interacts with our body.
Of course, pain play is something you want to explore slowly. You don't jump right into it; it takes time and practice to learn where your subs limits are, and how your sub responds to things like spanking. In the right setting, pain is both a powerful aphrodisiac and a tremendously pleasurable high. The key is to find how much is "just right."
Even during edgier punishments I have never caused her pain she didn’t need.
The difference in mindset between exploring BDSM with a lover and actually believing that women deserve to be harmed is as great as the difference between playing a hit man in a movie and actually being a hit man.

I am a strong believer in measured responses. When pushing limits, when demanding something for your own pleasure you need to be very aware of the capabilities of the mind you are dealing with. I have always been more than thoughtful towards my pet’s feelings, her emotional readiness towards a certain action and of course her overall state of mind. That sometimes requires me to postpone or re-plan, when I have set out to do something but in the end it does not quit pan out the way that it should be.
I am a lot more extreme in my application of perversion and kink, as such I have always had to “wait” for my sub to catch up. I am glad to say that this waiting period is over now. This is probably also reflected in the fact that I am starting to acquire more explicit toys. These toys will serve to physically express what my pet has known for a very long time, that she craves tight control and that she wants to be held accountable for her actions, very strictly so.

There is not one "right" way to behave that applies to all people all the time. If something adds pleasure to your life and to your partner's life, then it's not a bad thing, even if it is unconventional. And there's certainly no crime in taking pleasure from bringing your partner pleasure!