Both Wawa and Sheetz gas stations around where I live have free self-service air stations for topping off soft tires. Other gas stations charge. I don't pay for air if I can help it.
The Sheetz gas station right next to my place of work did a very good thing. They upgraded their air station to a very nice new unit. It had a digital readout and an easy-to-use hose. It was fully automatic. I set the pressure I wanted, and the hose started to blow air in controlled bursts. After I pushed the hose over the inflation pin on the tire, it filled it and apparently measured the pressure after each burst. When the tire was at pressure, the gauge beeped, and I moved on to the next tire. Couldn't have been much easier aside from having someone come out and do it for me.
Gas stations make almost nothing on the gasoline they sell. It's a very price-sensitive commodity with thin retail margins. The real money is in the store's items, which have a high markup. I can't imagine places that charge for air make much money on it. Sheetz and Wawa have it right: Offer that for free to bring people in. Not surprisingly, I tend to fill up there more, not just next to work but elsewhere too.
That makes me so angry when gas stations charge $0.25 – $0.50 for AIR! What a slap in the face. Half of them won't stay on for more than 30 seconds either, so if you have more than one tire to top off you're paying twice. I agree with you. Offer it for free and lure in customers to make purchases inside your store, your true earnings anchor anyway.