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 are jails: they do not allow the user to freely install applications. The image of the iPrison illustrates this issue.
These systems are platforms for censorship imposed by the company that owns the system. Selling products designed as platforms for a company to impose censorship ought to be forbidden by law, but it isn't.
iOS, the operating system of the Apple iThings, is the prototype of a jail. It was Apple that introduced the practice of designing general purposes computers with censorship of application programs.
Apple used this censorship power in 2014 to ban all bitcoin apps for the iThings for a time. It also banned a game about growing marijuana, while permitting games about other crimes such as killing people. Perhaps Apple considers killing more acceptable than marijuana.
Here is an article about the code signing that the iThings use to lock up the user.
Curiously, Apple is beginning to allow limited passage through the walls of the the iThing jail: users can now install apps built from source code, provided the source code is written in Swift. Users cannot do this freely because they are required to identify themselves. Here are details.
While this is a crack in the prison walls, it is not big enough to mean that the iThings are no longer jails.
Apple has banned iThing applications that show the confederate flag. Not only those that use it as a symbol of racism, but even strategic games that use it to represent confederate army units fighting in the Civil War.
This ludicrous rigidity illustrates the point that Apple should not be allowed to censor apps. Even if Apple carried out this act of censorship with some care, it would still be wrong. Whether racism is bad, whether educating people about drone attacks is bad, are not the real issue. Apple should not have the power to impose its views about either of these questions, or any other.
Game consoles are jails, too: no game can run on the console unless the console's manufacturer has authorized it. Alas, I can't find a article to cite as a reference for this fact. Please inform us if you know of one.