They do, however, have all the gas you can pump

... we don't carry that because there's no demand for it...

This is emphatically not a convenience store. It's a tiny booth where you pay for your gas. Anyone bright enough to peer through the glass would see that nothing's there, save for some motor oil. I'd get tired of saying "no" too.

Also, why should they expand their operation to carry products that a dozen drunks decide they want at 1am? Some demand does not equal LOTS of demand.

For those who are literally "dying of thirst" - there is a Hess station right across the street, and vending machines outside the booth. Self-service, just like when you pump your gas...