I think it depends on the specific situation. If I was planning to attend an event myself as a participant and work a shift while I was there, that's one thing. I'd pay the fee if that was the expectation.
But if my main reason for attending was to work as a volunteer - then I wouldn't expect to pay an admittance fee.
I also think it depends on the kind of event, the type of fee, how many volunteers you have, etc.
A question has come up at our school as to whether volunteers for an event should purchase a ticket as well. I've always purchased tickets for our silent auction, holiday parties, etc. even if I'm working at them. It never occurred to me not to. I figure it's a fundraising event and the purpose is to make money. Plus, I always get breaks here and there where I can shop, bid or whatever.
At a recent event, the chairperson asked me about it and said that some people don't. We agreed that it's probably something we should have a policy on, though I do not want to tackle that issue this year while I'm on the board. I'll let next year's board deal with it if they choose.