If you wanted it for therapeutic use, you’d have to jump through hoops and spend a lot of $ just to get little bit. Meanwhile, cops can have you sedated with it at will.
I’ve never used ketamine, but if it’s anything like similar dissociatives, an unexpected trip could be very disturbing. And on top of the other abuses from police, I can imagine the whole ordeal would cause life-long trauma (if you survived in the first place).