If FSF can re-license under a more permissive license, then FSF's assignment agreement implicitly allows the contribution to someday be used in proprietary software (since FSF could relicense to BSD or MIT, then someone else could take that code and incorporate it into a proprietary project).
So why doesn't anyone have a problem with the FSF's assignment?
The fsf's copyright assignment is also controversial (at the very least there is the issue that it adds an extra layer of bureaucracy for contributors).
Honestly, I think it only gets a pass because it is the FSF, a nonprofit entity that is probably the most vocal proponent of copyleft licensing out there.
So why doesn't anyone have a problem with the FSF's assignment?