Well I always say that trying to figure out movies is a lost cause. Movies aren't trying to build a complete universe, they are just moving the plot along for the desired effect.
The simple fact is the movie series has plot holes. Everything that happened on the nostromo was supposedly known by the company through transmissions by Ash..... but they just forget about it? Nope, it doesn't make sense. Unless of course you want to make a sequel where an entire colony was built in the worst possible place thus setting up a great movie.
Why would a company send space truckers to recover a potentially dangerous or extremely valuable Alien lifeform? They wouldn't, unless you wanted to set them up to be killed one by one making great drama.
Sometimes it just doesn't make sense, but it makes a great movie. Perfect example, my Alien game. The plot flies in the face of the movies. But I had to write a story to fit the pictures so I bent the plot to fit. People don't seem to mind.