Yes, people will get careless with autonomous cars, but don't they do it with regular cars too ? Cellphone, coffee, makeup, etc. So I disagree that autonomous cars have to be perfect.
Yes they do, but trying to do something else while driving is additional cognitive load --- they still have to be paying some attention to driving, whereas "it just drives itself" automation allows a complete diversion of attention. In other words, the automation encourages carelessness.