Maybe there were foreign support troops leaving them behind? Maybe it is from a private stash of criminally insame preppers? The P90 has as many reasons to be included/excluded as all the other weapons.There are weapons lying around all over the maps, as if people just left them there for our convenience. If you could only get guns from dead soldiers/cops I would consider agreeing with you on this one, but there are rifles, shotguns, pistols, all just placed on shelves, tables or just anywhere. If you really wanted the grade of Realism you're implying, this game wouldn't be much fun to most people, because they would just die instantly. One horde would be enough to kill every player, because you can't just "Push back" the weight of so many bodies with the push of a button. They would just rush you, you'd fall and be eaten. There is no real recoil in this game (especially considering the fact you're playing "normal" persons, probably without any training in the field) and you can trick the melee stamina restriction by just using the right mouse button to finish off hundreds of zombies with a single pipe in a single playthrough (already did, much much fun ^^).
I know there has to be some kind of inner logic and I agree with its necessity, but as far as the realism in this game goes atm, I don't see why especially the P90 would have no place in this game. I'm not asking for a vulcan cannon, laser guns or a rocket launcher, because I know THAT would be too much ^^