I’ve never seen a cop fill up at the gas station.
Loads of pros
Tax is not insignificant, but they could already write that off.
No middle man for what little profit comes from a gas station
They can buy cheaper in bulk
They’re not impacted by shortages and outages they control their own supply
They’re not impacted by quality issues if a gas station gets water in one of their tanks.
They don’t have to track individual purchases for every tankful
No lines
Officers not vulnerable while fueling when they can do it in private
In many places they keep fueling gear in the garage where they park the cars and just refuel that tank with a truck from their own supply.