The swipe prevents you from pressing an 'unlock button' by accident.
Imagine reaching for your phone, pressing the button by accident (in your pocket), calling somebody with the next mis-touch, then deciding not to use the phone while somebody listens from your pocket. Not a big deal, considering that could be a long distance call or you could be having a (seemingly) private conversation that shouldn't be listened in on?
Biting the hand that feeds IT
