There are separate commercial and non-commercial licenses listed in the repository. This may not be compatible with open source, but it's a reasonable licensing strategy overall and doesn't create a "need to pay" as most use would be non-commercial.

Creative Commons licenses are not designed for software, but they clearly encode this sort of provision for other creative works. Why shouldn't software developers follow a similar path?

I don't know where you're quoting "free open source lib" from. It's not in the OP nor in the project README as far as I can tell. In fact, the README explicitly calls it commercial software.