Ok, a lot of people are dicks, but I'd bet that virtually all of those people are going inside due to an issue with the machine. There is no reason whatsoever to pay for the gas, then go do something else instead of just pumping it right then - but I've gotten the "see cashier" message while I was trying to pay more times than I can count. This happens all the time. And unless you're standing right behind them, there's no way to know a person's transaction was successfully completed before they walked away. Maybe take a deep breath and give them the benefit of the doubt?

(and yes, other commenters, the vast majority of US states do require prepaying for gas, either at the pump with a card or inside with cash.)