Ciel, a D/s relationship isn't about the sub being a "doormat"
They are still equal in respect and human rights, and have the right to tell their Dom(me) if they did something wrong, although depending on the relationship they might have to do it in a very roundabout way.
The point for the sub is to give up all responsibilities, having to make decisions, etc, and letting someone you trust completely, and who knows you better than you know yourself, make these decisions for you. It takes a lot of stress off, stress that the person still has in the rest of their life even if at home they're a 24/7 sub.
Mind you, being a switch I have no direct experience with people who are in 24/7 relationships, or even relationships where the dynamics extent much beyond sex. But the idea that you need to stand up for yourself is still present, it's not about doing what the Dom wants as much as it's about letting the Dom ask you to do what you