[fr0sty]

> But you can replace the ECU with one of your choice.

As a counterexample: There are certain maintenance procedures for late model Audis that can only be done at the dealership. The diagnostic tool is connected to the car and then phones home to a central system to get vehicle-specific codes required to perform some actions. One such action, oddly enough is getting the car to recognize a new ECU[a].

[a] you can circumvent this by reflashing another ECU with special firmware to become a 'clone' of the original but this would be very analogous to having to 'root' your phone to install different software.