Games can have multiple genres.
As MP-Ryan said, FPS just means it's a first person game, and it's a shooter.
Nowadays you might not find it to be a very useful label but if you look a bit past the origins of the genre, you'll see that it differed very wildly from the games that were played back then, and that there weren't many FPSs. However, because there are so many different kinds of FPSs nowadays and there's what some might consider a sort of main branch of FPSs that have very similar characteristics, they confuse that main branch with the whole genre. Which is why we presently see games that have multiple genres or sub-genres and multiple labels describing them.
Long story short, the FPS label is overused because of the quantity of existing FPSs.