Because I want them to be controlled as thoroughly and safely as possible. I do eventually have to let them up and sometimes I have to explain to the nice judge why I tore the tendons in someone's wrist or faceplanted a guy in the parking lot.
Because I actually am a nice person who needs to make mortgage. The engineering degree is apparently useless in my area, so I may as well pimp out my black belt.
Because I would like to represent my art well and fairly.
"He attacked me, I grabbed him, he fell down while I was struggling with him and broke his arm. It all happened so fast, I was just trying to keep myself safe."
That is my prepared speech for anyone who asks me to explain myself after a fight.