Non-profit organizations still exist within the capitalist system. Just because they don’t pay out dividends to shareholders doesn’t mean they can’t be exploitative. If a non-profit makes a lot of money, the people who run it just increase their own salaries. Happens all the time. And yes, non-profits exploit labor all the time. Local arts councils are an easy example of this. Run an art show where the venue and food are donated. They take a fee at the door, ask for donations throughout, and require a cut of any art sold. If there’s a bar, they get a cut of that too. The artists who enabled the show to even exist did all their labor for free in exchange for exposure and maybe selling something. They see none of the take though. Sounds pretty capitalist to me.