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.
This page lists instances where proprietary software had a bug, and the developers converted it into a malicious functionality by trying to cover up the existence of the bug.
Price Waterhouse Coopers tried to suppress knowledge of a security flaw by making legal threats against the security company that found it.