There is a law, but as of yet, the only gas station ooperator who got prosecuted was an owner in Wisconsin (I think) who provided a discount to military members, their families, and veterans who showed their ID cards. Seems the station owner would provide a 2 cent discount, out of his prifits, to anyone who would show a military ID card. The judge who prosecuted the case stated he was providing an unfair price to non-military customers. That’s my recollection anyway.
Wisconsin has it's own unique law
http://www.datcp.state.wi.us/trade/business/unfair-comp/unfair_sales_act.jsp that makes it illegal to sell your gasoline at less than 9.4% times the amount paid for it. It is actually against the law to sell at cost.
Sam's Club, Costco, etc can not sell for less than the the 9.4% over cost, because of an archaic Depression era law. What happens is if any station gives “discounts” that are less than the price they paid they are breaking the law, and another station will turn them in. And that happens every year!
Our “minimum mark up law” is supposed to “protect” the Mom and Pop stores, but it only screws the consumers.
It is very, very rare to see more than a 4-5 cent difference at any gas station, so price shopping is not even worth your time. It is a scam that only hurts consumers by eliminating competitive pricing.