That's not to say that all OSS is like this, or that all software should be ran like this but there clearly is "free beer" and "free speech" in certain places.
You might say that I pay for the universities with my taxes or the other organizations by increased prices on goods, but it clearly is offered as a free service for all, even people not paying taxes or buying their goods.
If you can't donate there are lots of ways to help: add a feature, host a pkg repo mirror, clean up docs, fix bugs, add/manage a package, donate older hardware for compat testing, help out with community support, github issues. Telling people it is awesome also helps but maybe a more detailed why would go a bit further to promote it.
Would be more helpful though if corp OSS users who have their entire operations underpinned by financially undersupported OSS projects stepped up to help.