A couple of guys sitting around does not prove a crime and even if you had strong suspicion, you’re not a cop and letting them buy gas doesn’t materially support the criminal activity – the amounts are small and it’s the same service you offer everyone. If you’ll note that’s why I used examples which have lead to criminal charges where someone did something unusual which they knew supported criminal activity.
This is why anti-money laundering laws exist in the first place. Criminals would otherwise seek out a bank which doesn’t ask questions and the bankers would be hard to charge unless they were idiots and wrote something like “They are here for crime”. AML removes the ability not to ask or report certain types of activities, which is much easier to prosecute than e.g. proving that a banker should have known there was no legal way for a small Italian restaurant to be so profitable.