I used to volunteer for a local non-profit a few years ago.

From time to time, I would reflect on the fact that Microsoft and other commercial suppliers were getting paid for providing services to us, but I was expected to work for free.

Yes. Non-profits are more than capable of abusing the commons, the purpose of even small monetary requirements is to put a bound on that.

If Mozilla and Wikimedia can pay millions in salary to their CEOs, I'm sure that they can spare a few thousands for open source projects.

Yes. Not for profit does not mean they don’t have money.

With that logic why should non profits have to pay for anything at all?

For the same logic they are tax-exempt. There is a general consensus that their goal is the greater good (like developing sudo and such) and not the usual capitalistic good of generating more money.

Then again, you usual Friday outing of FANG engineers may have more money than some nonprofits too.

Yes.