Most mobile devices do stop you from installing a custom ROM and running your own code (or code from someone else). That kind of thing is what should be illegal. Likewise with e.g. banks requiring people to submit control of the computer they own to the likes of Google even if the device itself in principle can be put under the owner's control.

Locking down car repair tools is another obviously abusive practice that primarily benefits the manufacturer and harms the owner, justified through some weak appeal to security, yes.