The bit about the warranty is in your manual. Read it.
There is actually another company, Brugger & Thomet, you can use as well, but that is a European manufactorer and only Made in USA silencers can be sold in the US to civilians.
H&K is worried if you use a suppressor that is not built correctly it could break the polymer frame from all the extra weight hanging on the front of the gun. I suppose if you showed up at HK with a broken USP Tac the first thing they would ask is what suppressor you were using. If you choose to lie, that's your decision. I bet they would ask for a copy of the Form 4 if you claimed it was a KAC that broke it.
With all that said, I am running the USP CT on a Gemtech Blackside and haven't broken anything yet. Here's mine:
I am wearing sissy gloves because the thing gets damn hot after a few mags.