Freedom means having control over your own life. If you use a program to carry out activities in your life, your freedom depends on your having control over the program. You deserve to have control over the programs you use, and all the more so when you use them for something important in your life
If the users don't control the program, the program controls the users. With proprietary software, there is always some entity, the developer or “owner” of the program, that controls the program—and through it, exercises power over its users. A nonfree program is a yoke, an instrument of unjust power.
In outrageous cases (though this outrage has become quite usual) proprietary programs are designed to spy on the users, restrict them, censor them, and abuse them. For instance, the operating system of Apple iThings does all of these, and so does Windows on mobile devices with ARM chips. Windows, mobile phone firmware, and Google Chrome for Windows include a universal back door that allows some company to change the program remotely without asking permission. The Amazon Kindle has a back door that can erase books.
The use of nonfree software in the “internet of things” would turn it into the “internet of telemarketers” as well as the “internet of snoopers.”