From a subbie's POV:

It's good to remind a sub of the rules because we will forget things occasionally. Just a simple reminder will do, it doesn't have to be condecending. It also makes the sub feel that their Dom wants to make sure that they're doing their best.

We subbies aren't infallible. We do forget to do things. I forgot to do my journals two days in a row, honest to God forgot to do it. Sir forgave me, but let me know that if it happened again there would be consequences. If the sub just flat out doesn't do them, then I don't think they're being serious about it. If a sub genuinely forgets to do something, we generally feel awful about it and try to make up for it in any way possible. If the sub is just blowing off her tasks, she's ultimately hurting herself in the process because she is not getting the full experience that submission can bring.