Why King can't capture piece guarded by frozen piece

Is this a chess rule or just how this site is programmed?  Why can't a king capture a piece when that piece is guarded by a piece which is frozen in place when fronting its own king.  i don't understand this.

Because then your king will be taken, won't it?

I now see a whole discussion about this and no the knig can't be taken but the rule is that a king can't move into check.

Exactly. And what Evgeniy said applies in that position.

Of course king can't move into the check, I just tried to explain logic of the rule instead of giving a rule.

Good example. The point being that even though the bishop can't capture on d4, the king can't place it self/be put in a position where it may be captured, even if that next capture move is illegal.

I like McBurger with onion

Pretend the king is a normal piece just like a pawn but once the king is taken, the game is over and the person who took the king wins. If white is to move in the example given by tigerprowl, king takes rook. Bishop takes king before the rook can take the other king so it's a loss. The reason you can't move into check is pretty much it will end the game that way and is equivalent to resignation.

But there would be no 'after' - the white king's capture by the bishop would end the game immediately, leaving the white rook unable to move against the black king