Free as in free speech, not as in free beer. This has been a slogan of free software for decades. Charging money for your software is not in any way incompatible with open source.
The main important part of open source, regardless of license, is that you have the ability to run it locally to ensure that the code written and provided is what is used for your interaction with the code (since obviously a company or entity can lie and provide code that is different than what is used as the SaS.
Tailscale and Headscale come to mind.
of course not.
But nobody has successfully been able to charge for it, without additional things such as services/support etc.