With free software, the users control the program, both individually and collectively. So they control what their computers do (assuming those computers are loyal and do what the users' programs tell them to do).
With proprietary software, the program controls the users, and some other entity (the developer or “owner”) controls the program. So the proprietary program gives its developer power over its users. That is unjust in itself; moreover, it tempts the developer to mistreat the users in other ways.
https://www.gnu.org/philosophy/free-software-even-more-important.html