Sure, but that's immaterial. FSF (and many others; again, this is the fairly explici Constitutional rationale) views copyright bit as a natural right, but as a legal right which is desirable, within certain bounds, because managed properly it can produce certain public benefits.

This is at odds with your apparent view that copyright is a natural right.

> The source code isn't being published to begin with, so all of the usual arguments regarding length of copyright protection are kind of meaningless.

The lack of publication actually doesn't render the arguments for time limitations moot, only the argument that copyright in such works provides a public benefit (though that can be remedied by altering the terms of mandatory deposit to apply, instead of within a set time after publication, to apply within a set time after creation, and to adopt as a consequence of deposit failure forfeiture of copyright.)