All freedom is relative to the perspective you judge it. "Free" in this case is measured from the perspective of the software (and more specifically the source code), not the developer. The basic philosophy is that the easier it is lock up the source code and its development behind closed proprietary walls, the less free it is.

From the FSF perspective, your freedom to make the source code less free stands in direct opposition to the freedom of the source code.

The freedom prioritized is that of the user, not the developer working at a for-profit company. Copyleft software prioritizes users' freedom by attempting to prevent companies/developers from taking away their rights. (See also GPLv3's anti-Tivoization clauses: https://en.wikipedia.org/wiki/Tivoization that restrict companies to empower users)