In real BDSM (not this 50 Shades stuff) the bottom has most of the power. They are the ones who set limits as to what is and is not acceptable. They are the ones with the ability to stop play if it exceeds their desires. In fact, they are almost always the ones who initiate activities and choose what activities they want to experience.

50 Shades is a perversion of BDSM because it removes the consent entirely and shifts the driving power to the top, not the bottom (which almost never happens in real life) in real life bottoms and submissives hugely outnumber dominants and good dominants are in high demand.

Having said that, 50 Shades is a work of fiction, so treating it like was ever supposed to be a manual for a healthy relationship is a bit silly. There's a lot of people getting their panties in a serious bunch over a book which was written for a bit of fun and ended up getting a lot more exposure than the author probably ever bargained for.