Now let's move the same idea back a step. White has just played NxNc6; now Black must decide how to recapture. Which pawn should he use for the purpose—b7 or d7? If he uses the one on d7, his queen is exposed; the two queens would be aimed at each other. True, Black’s queen would be protected. But the protection is from Black’s king, and the position here would then resemble the previous one. White plays Bxf7+; Black would have to play KxB (or at least move his king to f8); QxQ follows for White. So in the initial position Black should recapture with his b7 pawn instead.
Lesson: exposing a piece because it seems safely protected by its king can be risky business. Can be—though of course it all depends on the details. If Black’s e7 pawn were instead on e6, for example, leaving his queen exposed wouldn’t be so risky after all: White’s bishop wouldn’t be able to capture on f7. Morals like the one just drawn refer to risks to consider, not rules to obey.