In my experience it's generally based on the Stallman concept of software freedom, in which the freedoms Free/Libre software is intended to respect are seen as rights, and the power that a proprietary software vendor wields over users is fundamentally unjust. This is the basic philosophy on which the Free/Libre software movement was founded, so it's not just a peripheral view. He outlines this pretty clearly here:

https://www.gnu.org/philosophy/free-software-even-more-impor...