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.
Google censors add-ons for Chrome.
Apple used its censorship system to enforce Russian surveillance by blocking distribution of the LinkedIn app in Russia.
This is ironic because LinkedIn is a surveillance system itself. While subjecting its users to its own surveillance, it tries to protect its users from Russian surveillance, and is therefore subject to Russian censorship.
However, the point here is the wrong of Apple's censorship of apps.
Apple used its censorship system to enforce China's censorship by blocking distribution of the New York Times app.
Apple censors games, banning some games from the cr…app store because of which political points they suggest. Some political points are apparently considered acceptable.
Apple banned a program from the App Store because its developers committed the enormity of disassembling some iThings.
Apple rejected an app that displayed the locations of US drone assassinations, giving various excuses. Each time the developers fixed one “problem”, Apple complained about another. After the fifth rejection, Apple admitted it was censoring the app based on the subject matter.
This particular political slant affects other Apple services.
Google censored installation of Samsung's ad-blocker, saying that blocking ads is “interference” with the sites that advertise (and surveil users through ads).
The ad-blocker is proprietary software, just like the program (Google Play) that Google used to deny access to install it. Using a nonfree program gives the owner power over you, and Google has exercised that power.
Google's censorship, unlike that of Apple and Microsoft, is not total: Android allows users to install apps in other ways. You can install free programs from f-droid.org.
The Nintendo 3DS censors web browsing; it is possible to turn off the censorship, but that requires identifying oneself to pay, which is a form of surveillance.