"Just what the heck do you think such a law would accomplish?"
One, it would mean people travelling along highways funded by federal tax money would not have to pay higher prices for gasoline due to idiotic local laws. I also want to annoy leftist wackos who to force their agendas onto the states by incredible contortions of the commerce clause of the constitution. At least the highway system that connects the states can reasonably be construed as affecting interstate commerce. My guess is that within a few years owners of gas stations that are further than the limit will lobby to get this stupid law prohibiting self-service gas sations repealed so they are not at a competitive disadvantage.