FireLance
Legend
Careful, there's a whole other potential can of worms here.The DM doesn't have to tell the players that the creature's AC is 30 for all of them, all the time. He can just mentally calculate "well, this character took Weapon Expertise for +2 to hit, so I'll make its AC two higher against him." How are they going to know that it doesn't have a printed ability giving it +2 to defenses under certain circumstances?
A good DM can deal with the issue.
If a player picks a feat thinking that it will improve his PC's attack bonus, but it never actually does, I think the DM would be better off banning the feat and letting the player get some benefit from his feat slot instead.
If the feat only benefits the PC during some fights but not in others, I think it is the responsibility of the DM to communicate this to the player, even if he only hints at it indirectly, so that the player gets a sense of the actual benefits of the feat and can retrain it out if he decides that another feat would be better.
Otherwise, the DM is effectively negating player choice or worse, deliberately misleading the player into making a bad choice. IMO, of course.