Parrots do not imitate camels

...and why should they?

Because they imitate other mobs too. 

And how, exactly, do camels qualify as a hostile mob, given that parrots exclusively imitate hostile mobs?

Who says the imitate only hostile mobs? In real life they imitate all noises. 

It's a widely known fact that parrots only imitate hostile mobs in the game as to act as a warning system. Some searching should turn up a handful of earlier reports regarding parrots imitating things they shouldn't.

Yeah, this is a feature request and not a bug. Parrot immitating in-game is hostile mob only.