The player doesn't have to cast a fireball when that is the answer to the challenge, the character does. The player doesn't have to use his nigh superhuman strength to wrest the black mace from the demon when that is the challenge, the character does.
Why is this different?
lets look at it another way... if the player were a smart fellow and the character and int 6 wisdom 6 barbarian bruiser, if the player saw thru the puzzle right away, should he tell everyone or should he decide that "in character" his barbie knows squat about "weighing no puny balls" and sit back and have his barbarian show lots of frustration, griping, and grumbling?
i had a case something like this a couple decades ago when i was playing for once where a Gm ran a dungeon for everyone's "favorite characters all together" and it turned out to be a "nothing works but solve the puzzles" magic null thingy "cannot leave" where every wrong answer cost you "permanent hit points" and such. of course, the questions had nothing to do with "in character" at all. Some were TV show catch phrases (one specifically set to match up to a player's favorite show, but that player (GM's brother!) did not join the campaign so we had no clue), one was the GMs social security number, and so forth. Some were significantly vague as to guarantee several failed tries (and lost "permanent hit points") such as "to pass this door you must shout the barbarian's oath" which had as the correct answer "lords of light" from Thundarr the barbarian which only the Gms brother watched. of course, our knowledge of Conan, kull, and several other barbarians cost us quite a few hit points.
That one did not end well either.