Funny thing is, lots of classes in 3e have alignment restrictions, which are every bit as severe as paladins, but they are never enforced.
A barbarian who acts in a lawful manner loses his Rage ability and cannot advance any more as a barbarian. So, if the barbarian player plays his character as always following orders, never acting impulsively, creates elaborate plans, etc. he should be a "fallen" barbarian.
But, it never happens. For some bizarre reason, whenever a player puts Paladin under class, the DM suddenly feels empowered to force his view of how that character "should" be played on that player. But no other class ever has this enforced. It has always boggled my mind.
Sounds like personal bad experiences with (crap) DMs (bummer), as most paladin complaints do seem to be.