MenuetOS also suffers this problem. The 64 bit version is closed-source/non-Free, but available free of charge. Great way to limit wider interest in the project, and they're not even making bank. They get to spite the people who'd want to fork the project, I guess.
The D programming language also had this problem for a long time (closed-source/non-Free compiler backend), but thankfully no longer.
Tiny correction: the reference D compiler's source code was available, but its backend was not Free Software (it was non-redistributable). This was not by choice of its authors, but due to a historical IP/licensing snag. Today it is fully open-source.
Certainly in the case of D, adoption and real-world usage was/is a goal. Perhaps less so for MenuetOS and SymbOS, but even 'pure' hobby projects tend to enjoy public interest. They don't just live on someone's hard-drive in a basement somewhere.
That D's licensing issues are now gone, certainly benefits D.
Even if adoption isn't the goal, making it FOSS might still make good sense. Suppose the goals are novelty and technical achievement. More interest means more support in developing their novelty OS, no?
The D programming language also had this problem for a long time (closed-source/non-Free compiler backend), but thankfully no longer.