Lowering prices isn't illegal. There are only certain very limited circumstances where selling below the market rate can potentially be illegal. One is dumping, where foreign companies sell at artificially low prices in an attempt to drive domestic competitors out of business. Another is when a domestic monopoly (or near monopoly) does the same. But if a fuel retailer wants to sell gasoline at $1 / gallon less than the competition then that's totally fine (although local law enforcement might be annoyed if that causes a traffic jam).