Sure they are scifi films, they obviously are set in space. Alien, as a single genre descriptive, is a horror film though (mr Ridley says Terror but I think he's nit picking
). Just because something is incidental to the story, it neither negates other aspects of the film, nor increases their importance in the film just because they are there. Films would be shallow and pretty pointless if they were that 'dumb' (and too many films are going down that route nowadays
The problem with the scifi genre descriptive, is it merely denotes science (which is pretty crappy and without many constraints), so a lot of stuff can be classified as science fiction, be it Reanimator or Frankenstein, Star Wars or the Hulk. You just have to use your noggin though when applying a limiting singluar genre descriptive to titles.
The majority of the hits to this site go via the mainpage btw. Feel free to make an alien collection of all 8 flavours, but its pretty pointless recycling those links again
ps. why get into this again, just curious?