Maybe I missed something but why is it the Aliens put up with so much sh*t? Their weaponry was beyond superior to that of ours. A few of them in that Iron Man-like suit could easily take over a country within a few days.
I think they say something towards the beginning of the movie, when they are interviewing people, that the aliens didn't have any motivation or something like that. They assumed because of illness.. They were sick and near starved when they were found and just like human nature if you constantly do something for someone they wont do for themselves.
It's implied that the aliens are caste based (in the opening prologue), and the leadership caste basically has died off. So, they're the equivalent of leaderless worker bees. You can probably assume the lead alien (Christopher, IIRC) in the film is a member of this higher caste, as he demonstrates greater initiative and far greater intelligence than the other prawns we see.