So a game that is different from MW2 in nearly every imaginable way, is a copy of MW2 because they share a few of the world's most commonly used firearms?
You're missing the point.
KF is an excellent game for unusual weapons, since it doesn't have to fulfill any accuracy/aesthetic requirements, such as how WWII games require WWII guns, and Modern Warfare requires modern guns.
So, instead of taking advantage of this opportunity for some truly unique and original ideas, what do people do?
Why, regurgitate the same handful of guns they've seen in nearly every FPS of the last 10 years!
People are boring, unimaginative morons that aren't comfortable unless every game they play is a rearrangement of the same dozen set-pieces every AAA game title uses for that very reason.
Why do you think the Ideas and Suggestions board is chock-full of "ideas" like:
- OMG we should have air/mortar strikes! Just like BC2/BF2/MW2!
- Totes awesome idea for <gun that does the same thing as an existing gun>! How about we add <gun from CS:S/MW2>?
- We need molotovs/helicopter extraction/specimen that does X! No I don't realize that this game isn't L4D2.
Also, FYI, there were likely far more Sterling submachine guns manufactured than MP5s and certainly many times more than UMPs. The latter two are really only currently used by over-funded police forces that have never had to deal with a threat more significant than a crackhead with a knife or a rabid dog.