Interesting line of thought but if they can get here it means they have the technology that is so much better than ours that if they wanted to destroy us they would have done so very quickly.
I watch some Star Trek and in one of the shows they expressed a theory that you are not supposed to interfere with a lesser species evolution. This is probably what is happening with the aliens they are here to observe. But they do not want to play God. I would also argue that they are probably morally ahead of as well as technologically ahead of us.