And flick your cigarette butt into it as you drive off!
With regard to having a trace of ethanol in the gas, it's probably going to happen if you're filling from a pump that dispenses E10 and ethanol-free from the same hose. I'd guess there's a half gallon to a gallon in that hose? But worst case, you're talking about a percent or two; bring your lawn mower can if you're worried about it. It's hard to see Polaris using this as a basis for denying warranty claims - I think there are too many ifs that would be hard to prove for this to bite more than the odd guy or two - but going back to when it was the other way around, I'd bet a good lawyer could make them pay unless they could prove it was more or less E10.
Now they have an even more difficult case, because they originally told you to run eth-free, and other than catching fire, they have no argument for why that's worse. Maybe they have a case if they can prove you were running E10 in premium mode is all. Ultimately, my hope is this is just CYA for the one dumb@$$ who sparks a fire and they won't bat an eye at anyone who continues to run premium. Time will tell.