We don't have concession stands for kids sports here. I guess I'm not understanding why a child's sport would need to raise that much money, but I accept that they do.Having said that, I would have read the initial contract you describe as "you pay this fee, and get it back if you VOLUNTEER to work", and given my schedule, I would have said "OK, keep my $25".
I work a part time job (in addition to my regular job) that is hourly as a consultant, and schedule my hours pretty closely around my child's sports. A 90 minute shift with a 60 minute commute would mean 4 lost hours for me, which is way more money than a $25 or $50 fee. Would I complain about it? No, but I would go into it knowing that I was planning on paying the fee. If my $100 sports registration suddenly turned to $500 registration becuase of lost income due to a change in policy, I would not be amused.
Now, if she knew upfront that concession stands were mandatory, then she could have chosen another sport for her child. But if all the information she had was "pay $25 or volunteer" then I think she gets to choose between those two things.