Nonfree (proprietary) software is very often malware (designed to mistreat the user). Nonfree software is controlled by its developers, which puts them in a position of power over the users; that is the basic injustice. The developers often exercise that power to the detriment of the users they ought to serve.
Here are examples of proprietary systems that impose censorship on what their users can access. We have a separate list of proprietary systems that censor installation of application programs (we call them “jails”.)
Selling products designed as platforms for a company to impose censorship ought to be forbidden by law, but it isn't.