People aren't smart. They like full-service, and they know it'd disappear if it wasn't the law. What they don't realise is that the reason it'd disappear is that they don't like it enough to pay extra for it.
The stations themselves might not be against full-service, either (at least away from state borders.) It's an extra cost, true, but it's paid by all of their competitors, too, so it just drives prices up. Demand is pretty inelastic, so profits aren't much affected. This might change with electric and fuel-efficient cars becoming more common, but it's hard to think long-term when you're selling oil.