Free Drinks When Switching Seats?

I was recently on a flight where a couple got split up due to a premium upgrade, and was asked to switch seats (informally by the couple - not a FA) into premium so they could sit next to each other. I was happy to have the extra legroom, so I agreed. Then, during the drink service, the FA asked what my original seat was, and charged me for the alcoholic drink I ordered.

I'm wondering what the policy is regarding this type of thing, because I was a bit surprised the FA would keep track and even care about it.