please don't conflate private with nonfree. most private code is actually free software, since all the users have full control over the computing done with it. that's a bit of a corner case, because "all the users" turns out to be the developer alone
I totally agree that a developer is entitled to keep their code from being used by others; make it available for others to use is a freedom, not an obligation
what is unethical is to make it available to others in ways that do not respect their freedoms; that enables the developer to subjugate the users. even if the developer never abuses that power, the fact that it's there is harmful and unethical enough for us to reject and recommend against such behavior