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 by the company that owns the system. Selling products designed in this way as platforms for censorship ought to be forbidden by law.
iOS, the operating system of the Apple iThings, is a jail. Apple has used its power to censor all bitcoin apps for the iThings.
Here is an article about the code signing that the iThings use to lock up the user.
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.