Knowledge should be free, but that can't be treated too literally. Not a unique case of this kind of phrase. If we're doing capitalism people have to be paid somehow, and when people say "free" they don't mean "absolutely". I mean, speaking of open source, consider "free" software.
Open source licenses are almost entirely unrelated. They're strictly a hack around the copyright system, and not only that, they literally do nothing other than grant you rights you wouldn't otherwise have. Talking about open source is mostly a distraction. When people say knowledge is free they almost always mean access to knowledge. Open source grants people access and more.
People are not mad that they can't just steal things, they're mad that access to things is tied behind massive gatekeepers (essentially indefinitely...) that essentially exist to continue to enrich themselves while somehow almost none of the money makes it back to the authors, and is sometimes completely untethered from where the money comes from that funds the works to begin with. You can't just freely navigate, search through and consume information, it's all tied up behind various pay walls and monetization schemes while authors starve anyways.
We could have a more equitable and reasonable system that allows broad access to knowledge while providing some approach to monetization that is reasonable for both people seeking it out and people consuming it. There's little point in trying to enumerate the number of ways it could be done. We already have a system for taxes, we already have seen commercial schemes like Spotify, you could slice it thousands of different ways. Plenty of pros and cons. I'm just saying it could be done and we know it could be done.
But it can never work if all media and knowledge dominated by rent seeking gatekeepers standing in the middle whose primary purpose will always be to enrich themselves first and foremost. They will always want to get more and give less, because that is more or less their fiduciary duty.