Why not, though? If a NPC charms my PC and forces me to solve a crossword, why is a knowledge/INT check not forced?That's wrong. First off, a saving throw is forced on the target. Knowledge checks are not.
But it is the PC who chooses to fail the save - in the fiction, s/he does not attempt to avoid the magic.the player is just deciding what his character absolutely does not know. Similarly, you can have a PC intentionally fail a save. What the player could not do is decide absolutely what the PC does know. Like saves, all he can do is roll to see if he succeeds at the knowledge check. I treat both situations the same.
But knowing something, or recalling some fact, isn't voluntary in the same way. The players choice not to make a knowledge check doesn't correlate to any similar choice being made by the PC in the fiction. It's purely metagame.