A long time ago, I used to work at a gas station when post-pay was allowed. All of the pumps required the cashier to activate them when somebody picked up the nozzle, but it was just expected that you hit the button to allow them to pump first. Most of the time, it's just a normal transaction. Sometimes, you realized some asshole stole gas when the next guy was at the pump.

Enough of this happened, especially during periods of high gas prices, that my boss would say "prepay is required now", and we'd tell them they have to come inside first. Eventually, paying with credit cards was so popular, and cash transactions were so rare, that nobody allowed post-pay anymore. There wasn't a point, when the risk was people would steal gas, and it's so easy to just stick a CC in a slot.

So, yeah, there's locks on the pumps, but if post-pay is the norm, it doesn't really matter.