Changing tack slightly. OP. As others have said, they probably paid for the stall, and the summer fair is about making money.
Having in previous years been the main person organizing the summer fete, it was a BLOODY lot of work. I was in tears several times. I felt isolated and alone (cos I'd send out a group email saying (for example) "xxx has happened/needs to happen and I don't know anything about it, can anyone advise?") and no one could, so I got no replies to an email sent to 15 people, and had to deal with whatever the issue was on my own, and having to guess and make judgement calls. It was awful. I started preparing in April and the event happened in July. I was spending hours in the evenings on top of working. True, I was chair the year after a lot of very experienced people moved on from the school, so it's not that bad for everyone, but I would say that I find that the burden of organizing these things lands on the heads of a very small number of people.
So I really wouldn't have been impressed if someone who'd done (presumably, as you were surprised to see the stall) very little towards contributing to making the event happen, rocking up to complain about an element of it that didn't fit with their personal ethics. I would have been polite, but I would have been thinking "well join the bloody committee and help us, then!".
And, btw, I was also a gvnr when I was chair of the committee.
Not meaning to be heated or aggressive, just to give you another perspective.