Why would they care if you're sitting on the bike while pumping gas? I do it all the time, I didn't know it was an issue.
Because you could loose your balance and fall with a live pump on or you could spill gas on yourself or your engine or whatever. Just get off the flippin bike and stretch, what's it going to hurt. Seriously I probably have the smallest tank on a bike and I've never found the need to get that few extra millimeters in there so I can go those few extra meters.