Nothing is for free, not even a slave.

And to own a slave, a Dom has to earn her/him. That's the natural way. Earning a slave includes, earning the respect, trust and willingness to submit freely.

In general, being a human we respect everybody, that doesn't mean respect comes unearned. The qualities and character of a person brings about the respect He deserves. Same is the case with trust and the wilingness to serve that person.
There is no other way around for the non-coercive bdsm relation.
For a coerced a forced relation, well it can not be said a bdsm relation, it would rather be called as a case of exploitation, a felony.