There is no preferential treatment. Cops need to get gas, they are going to stop to get gas anyways. They are not being given a discount for hanging out in gas station. This won't even really affect cops whom are on duty. On duty cops don't pay for the gas in their cruiser so they won't care about the discount. Cops heading to or from work will be more likely to stop there. There is no getting a "tip" for doing their job, it is getting a discount somewhere in their off time.