Can you blame them? I know if I built and sold a gun, based on what I've seen some "reloaders" do, I wouldn't want some of them shot in it...at least as long as the warranty period.
I'm all for reloading, do a lot of it (mostly shotshells), and at least attempt to build loads based on recipes, but that doesn't mean I'm beyond "educated" component substitution or, on a handful of occasions, experimentation.
That said, I wonder how hard it would be for a manufacturer to prove reloads, say, if you cleaned one really good before sending it in for service?