OK, I may very well be wrong about this, but...
... I'm sure I remember reading somewhere (most likely somewhere here on the board

) that the ortho prepares each tooth with a thin layer of the bonding agent, then bonds (or glues) each bracket to this thin layer of glue. When s/he goes to remove the bracket, s/he uses pliers to squeeze or flex the curved rear sticking surface of the bracket, which breaks the bonding agent between the bracket and the tooth, 'popping' the bracket off without damaging the tooth.
Of course, I may be wrong, it's happened before..
For what it's worth, I've had 2 x brackets removed completely painlessly, but it was only about half an hr after they went on...