They that are bound must obey.

A person constrained by another's power has limited freedom to refuse. The proverb presents obedience as a consequence of that constraint, rather than necessarily as a willing choice.

It can also express the broader idea that a binding obligation limits what someone may do. In either reading, the emphasis falls on the lack of a practical alternative.

Use it to describe a situation in which someone must comply because they are under another's control or subject to an obligation. It can convey resignation; it need not imply approval of the authority involved.