I can think of no plausible reason why they threw in that the Prawns would cause a train wreck "for fun." The aliens also seem to murder humans while committing theft.
It seems to me after the first few train wrecks, any sort of treatment of the Prawns, certainly relocation to a more secure spot, would be justified.
Could it be that the train wrecks and even murders were just propaganda? If not, if the Prawns (not all of them, for the scientist seems to be a reasonable guy) are really this dangerous, why would the UN or foreign countries be trying to dictate to the South Africans how the Prawns should be treated.
Again, the train wreck thing does not make much sense to me. The theft, given that the Prawns certainly live in difficult circumstances, sounds reasonable but if the aliens are intelligent, as even the workers are, why would they jeopardize their position in a strange place by wanton violence?