If I owned (or managed) a business like Starbucks, I'd certainly give free coffee to officers on-duty. It's a kind gesture, and it CAN build a good relationship and some goodwill to cash in when you might need it. That said, it's a little presumptuous for the officer in question to assume his drinks will always be free. If I were the cop, I'd assume I needed to pay, and be thankful and polite when/if the drinks are free.

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)