I spend quite a bit of time at social things where we're buying rounds or either teas or drinks. I don't mind at all, I think it's swings and roundabouts and over a lifetime ill have accepted the same number of drinks as I've bought, even if they're not all for the same people. However, I do notice the people who never step up!
I recently went on a thing organised by a volunteer. Just three of us went to the pub afterwards, the (male) volunteer, me and another man. I said I'll get these as we approached the bar and I was very happy to do so. The volunteer and I with had a pint of beer, the other man had a small coke, all good.
When it came to the next round the coke man bought it. Now, in my circles you don't let a non drinker buy drinks and it should have been the volunteer's round, but maybe as acknowledgement for his volunteering, that's OK?