That's not really true, things can become de-facto standards even if they're proprietary, especially if the actual status is in doubt. Many developers don't think about the long term consequences of licenses when they start using stuff, especially if it is useful and free of charge.
Linux is a perfect example of a proprietary API (Unix) which is in very widespread use, which was cloned by FOSS. Most of the Unix command line utilities were explicitly cloned by the FSF to be compatible with existing shell scripts on Unix.
Adobe made Flash proprietary and it became incredibly widespread, so widespread that if you turned if off, many websites would break, despite the fact that it had numerous performance and security issues. Flash made it to something like 97% penetration of all Web clients (prior to Apple iPhone) People have cloned Flash (hAXE for example), but there was really no reason to do it, because Adobe offered the player free of charge.
WINE is in widespread use. Samba is in widespread use. Lots of Website REST APIs have been cloned.
Adobe PDF was proprietary and widespread cloned until in 2008 it became an open standard.
Cloning APIs is a time honored tradition for "embrace and extend". Microsoft even cloned proprietary APIs, and DOS itself started out as an unauthorized clone of CP/M. Where would Bill Gates be today without Seattle Softworks?
I hate having to resort to the tired ole idea of rich people having gotten rich by theft, but in many cases, it's quite true, that Gates and Jobs both freely copied, cloned, and modified other people's stuff, got stupendously wealthy off of it, but somehow, it's wrong for us to want to practice their methods?