I've been working at a nonprofit for the last year. We simply would not be able to function without exploiting volunteers for free labor. We are quite literally in a huge conflict around how to restructure roles and staffing with the budget we have because the amount of work that needs to get done greatly exceeds the amount of money we can pay folks. Our most dedicated volunteers are essentially taking on part time jobs for free because they care about the people, the work, the movement, and the community. And from what I've seen, this is the norm.
Everyone is frustrated and confused when I say this is exploitation. Volunteers are not bound by employee contracts they say, so they can leave at any time. Plus, they argue, their livelihood isn't dependent on it. But nonprofit volunteers are coerced into extra work by holding their community over their head. People volunteer to find fulfillment and build community, and those are needs a nonprofit exploits (my nonprofit also strung volunteers along promising eventual paid roles they did not follow through on). Nonprofits convince themselves and others they are exempt from worker's rights regulations because their small budgets mean they cannot operate without exploitation, and how dare you say a nonprofit shouldn't function with how important their services are.
It's all to say: It's illegal to coerce a staff member to work extra hours without paying them. It's completely legal to coerce a volunteer to work extra hours without paying them. The model sucks.