Yah, smgcon has it right. It's all about American law. The USC is almost a tribute to anti-"Assault Weapon" laws in that country. As they are made in Germany, a semi-auto UMP-type gun simply would not be allowed into the U.S. for regular civilian end-users unless there were major changes. Just like how the various semi-auto MP5s and similar firearms have not been allowed to be imported into the U.S. since the 1980s. Have a look at this to see how history is repeating itself:
http://en.wikipedia.org/wiki/Heckler_&_Koch_HK_94
Hence, with the USC, the non-pistol grip, the non-blackness, the 16 inch barrel (which comes from a different, older, U.S. legal requirement), and the 10 round mags. At least HK has backed off on the colour now.
Of course in the U.S., the USC mod'ing "industry" has gotten pretty big, although they have had some adverse rulings from ATF on some issues:
http://www.top-notchparts.com/
The USC is pretty funky, it's true, and we are unlikely to ever be able to get around the 5 round mag issue with this firearm in this country, but still... my mind keeps coming back to the idea of getting one. If you want an HK long arm in this country, the SL8, USC and SLB2000 are it. The single-digit numbers of single shot G36's and UMP's that came into this country are clearly a screw-up that will never be repeated. Any talk of getting "civilian" version of the MP7 or other such firearms is pure fantasy in my view. Even the civilian oriented FS2000 and PS90 from FN seem to now be all but a dead issue.
For myself, I'm seriously considering doing a USC project gun. The barrel cannot be cut to 8 inches like an UMP, contrary to what some people have said on this board.
(see "Prohibited firearm" under section 84 of the
Criminal Code)
The best you can do is put a shroud on it that makes it look like a can. They sell them in the U.S. You can also get an aftermarket Lower/Stock that gives you the UMP's pistol grip and folding stock, and you can get the rails and foregrip.
It's a lot of work and expense though. I may just get a RRA 9mm AR-15. At least I can get 10 round mags for that.