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.
A tyrant device is one that refuses to allow users to install a different operating system or a modified operating system. These devices have measures to block execution of anything other than the “approved” system versions. We also refer to this practice as tivoization.
Some LG TVs are tyrants.
Apple arbitrarily blocks users from installing old versions of iOS.
The Apple iThings are tyrant devices. There is a port of Android to the iThings, but installing it requires finding a bug or “exploit” to make it possible to install a different system.
Some Android phones are tyrants (though someone found a way to crack the restriction). Fortunately, most Android devices are not tyrants.
Microsoft accidentally left a way for users to install GNU/Linux on Windows RT tablets, but now it has “fixed” the “error”. Those arrogant bastards call this “protecting” the users. The article talks of installing “Linux”, but the context shows it is really GNU/Linux that users install.