I'm torn on that issue. Either way, you cheat somebody. I think I just let them buy me Diet soda more often than refuse the offer. That way everybody gets a fair shake. Sometimes I order a shot of Keluha for my coffee, and then after a sip ... I sneak in the kitchen and dump it out, and pour a new cup. I hate to see them waste money, but the bar needs to make a living too. Double sided coin? By now, most folks realize that I don't drink at work so it's not as much of an issue as it once was.
Strange behavior at night-time, huh?