Google and the rest are pretty much the same.
Returning to the soft drinks analogy, why don't they sell RC Cola?
They buy cola by the crate and there is a better margin selling a case of the presumably more expensive crate of Coca Cola.
If RC Cola retails at 50c and Coke retails at $1 then they are not going to stock RC Cola. The supermarket are in a different situation, they are selling multi-packs and they have their own brand stuff, the RC Cola and the real Coke product for their customers, Coke having the best shelf.
If the gas station had demand from customers for RC Cola then they could stock it. However, 50c of revenue would be walking out the door with every sale of a cola-style beverage. Even if RC Cola have better margins when buying at trade that 50c difference is a big one.
The supermarket can increase footfall by offering RC Cola at a competitive price, people coming in for it might buy other stuff where the real money is made.
With the gas station a reduced price RC Cola offering is not going to increase footfall. Plus gas only has slim margins to it there is no elasticity there. The beverage and candy options are therefore really important for profit. Hence only certain products can be sold, the business cannot afford to stock anything that gets sub-par revenue or put that tap there for people to obtain limitless quantities of wholesome tap water.