It's an alien movie... I mean my goodness for all we grip we're getting a lot of content in movies, comics and games. Why are so many of us complaining about this?
Remember, Prometheus started because Ridley wondered why no one ever asked what that space jockey was. So he decided to tell a movie about it and Scott always thought the alien was a bioweapon. What started off as a generic alien movie branched out into epic territory and got the name Prometheus. However the idea got way the f**k too big and out of hand, so we got what we got in Prometheus. Yet the man simply forgot that the space jockey is in fact IN alien. It's the start of all the events in the alien universe. Yes, Prometheus is a prequel. Adding Alien to the title merely brings it home to it's roots.