I assume a gas station can charge whatever they want. Even stations of the same "brand" are separate businesses... here in Los Angeles you'll see Shell in Beverly Hills charging $1.00 more than Shell in the valley. No different than the first and last gas station before a long highway run in the middle of nowhere charging more for "convenience" really. There's a moral consideration (probably still not a legal one) when you're talking about hiking prices during a hurricane evacuation or similar, but this is just business as usual.
But in your picture, it's possible the station is still selling regular from a prior shipment that cost them less. While premium is a new shipment with a higher cost to them, and now you. I believe some stations only have a regular and premium underground tanks, with mid grade being mixed at the pump.
IF this is the case at that station, they're doing the folks that can use regular a solid by not charging a higher price on gas they haven't yet had to pay a higher price for themselves, and missing out on easy profit. They would be making good money on the mixed mid-grade, but that is their right I suppose.