legal move?

Ohgod it's this thing again. I hope this time it's a genuine question and not a troll. Anyway, to answer the OP...informally speaking you can think of it as the idea that "the player whose king gets captured first loses". So in this case, if the king took the bishop, Black would "capture" White's king one move before White can "capture" his, so White can't capture the bishop.

Kxh6 is not allowed, why can't King capture Bishop? Kxh6
g7xh6 would expose Blacks king to check by Rook@g4 and not be allowed, therefor eliminating capture of White King which should leave Black Bishop unprotected from attack from any piece including King.
Because of the FIDE laws of chess,Articles 1.2. , 3.1 , 3.9
http://www.fide.com/component/handbook/?id=124&view=article