You forgot to add the corrosive sulfuric smelling additive that is put in propane. It is more likely the reason for the warning. since as you pointed out that GG and Prop are essentially the same. Only each has a different additive. As someone also pointed out they can't guarantee that a person is even using silicone at all when they use prop.
Rather than phony pony about KWA or ripping off companies, I'd point out that the warranty would likely expire by the point you actually started getting propane-induced issues, and that KWA parts are so cheap that it's not a huge issue if stuff breaks.
"Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.”- Albert Einstein