So you're saying it's wrong of me to hide my maps and monster statistics?
Nope. As I've said, repeatedly, here and elsewhere, there is a difference between actively
preventing your players from acquiring information about something, and
not openly stating absolutely everything.
It's perfectly fine to keep maps concealed, or partially concealed, up until such point as the players
should in fact know the lay of the land. When their characters actually do in fact observe, for instance, the arena they're fighting in, do you keep those maps concealed, so you can secretly alter the layout without the players knowing that it's been changed?
It's also perfectly fine to not speak
openly about a monster's statistics. To use an old phrase in an actually correct way: the proof of the pudding shall be the tasting thereof, or in this case, the proof of the monsters shall be in the fighting thereof. Once the monster is in actual play--"minis hit the map," for those who use such tools--then its numbers shouldn't change except by some kind of observable in-world action (whether your action or the players). Because those numbers will be
tested ("proved" in the archaic sense) by the actions that occur in combat. Initiative, hit points, attack bonus, damage dice, save or defense values, etc.
The course of a fight is heavily controlled by how much information you can gain about your opponent, and how well you make use of that information. Changing that information on the sly rips the carpet out from under the players. They now cannot trust the information they've gathered--any part of it could be flat-out mistaken. Bad intel produces bad choices: garbage in equals garbage out.
Edit:
As for the "people only do it for player enjoyment," well, that's kind of a problem, isn't it? Because the secrecy is seen as a critical part of making it enjoyable in the first place. I can show people the full map of the area they'll be adventuring in, and it won't make a substantial difference in their enjoyment. I can show people the statblocks of the monsters they fight, and that won't make a substantial change in whether the fight is enjoyable or not. But numerous people--the plurality of the voters in this thread (not that I think any more highly of forum polls than you do, probably less!)--being told, before or after the fact, that the DM fudges rolls in combat WILL reduce their enjoyment. Reduce it enough to make them leave the game, even! It certainly would for me, if I found out the DM had been fudging and
especially if I found out they'd been fudging after I'd point-blank asked them, "Do you fudge rolls?" or "Did you fudge any of the rolls in that combat?"
So even by the "for player enjoyment" standard, there DOES seem to be a difference. Merely useful for enjoyment, vs. legitimately critical.