Proprietary software, also called nonfree software, means software that doesn't respect users' freedom and community. A proprietary program puts its developer or owner in a position of power over its users. This power is in itself an injustice.
The point of this page is that the initial injustice of proprietary software often leads to further injustices: malicious functionalities.
Power corrupts; the proprietary program's developer is tempted to design the program to mistreat its users. (Software whose functioning mistreats the user is called malware.) Of course, the developer usually does not do this out of malice, but rather to profit more at the users' expense. That does not make it any less nasty or more legitimate.
Yielding to that temptation has become ever more frequent; nowadays it is standard practice. Modern proprietary software is typically a way to be had.
As of April, 2017, the files in this directory list around 300 instances of malicious functionalities, but there are surely thousands more we don't know about.
Users of proprietary software are defenseless against these forms of mistreatment. The way to avoid them is by insisting on free (freedom-respecting) software. Since free software is controlled by its users, they have a pretty good defense against malicious software functionality.