The trailers show Space Jockeys... that doesn't mean the movie is a straight prequel to Alien, it means that it's a movie about Space Jockeys, plain and simple. That's another thing Ridley Scott said in an interview, that none of the movies (Poor comics, always disregarded) did expand upon the idea of that misterious extraterrestrial. The trailers themselves show Space Jockeys, the movie's characters, the movie's technology, the movie's story, the quest the characters are after, it shows its own movie... its conections with Alien are subtle (The way title appears, the designs by Giger...), it was the word of mouth that strengthened the idea of it being an Alien prequel.
Some may think that because it has Space Jockeys it has to mean that it will be about the origin of the aliens, when actually the idea that the Space Jockeys created the aliens is just an assumption, made for the lack of anything better, there was nothing in the movie that suggested that (In fact, if one analyzes the evolution of the script, and considers certain concept art and certain deleted scennes and the original ending, one may conclude something different...).
And as far as I know, the trailers haven't been all over our faces about Weyland. The company featured prominently on the viral campaign, on the internet. One had to do research to find out about Weyland.