The concern here is that manufacturers block access to parts, diagnostic tools, etc so the only way to service the equipment or vehicle you have purchased is through the manufacturer or their designated representatives. The manufacturer could implement a process where if "un-authorized" parts are detected the vehicle is rendered inoperable. This is no different from ink jet makers implementing those stupid chips on their cartridges to force buyers to use only "genuine" ink.
This is about protecting revenue streams from future service and limiting competition.
Sounds like pure conjecture.
Here's the opposite side of the coin. We are on the brink of self-driving cars. Right now, cars have multiple computers that must communicate with each other in order for the car to function as it was designed. If I was an auto manufacturer, the last thing I'd want to deal with is Joe Hacker bringing his car in because he's got it all fucked up and demanding that it be fixed under warranty. But that would be just an annoyance. Safety is a much higher concern.
Here are some very important processes that our cars do for us on a daily basis.
Emissions
Traction control
Anti-lock braking
Electronic stability control
Braking
Steering
Suspension
HVAC
Instrumentation
Powertrain
Transmission
Airbags and the degree to which they deploy
Some cars can apply the brakes if needed
Some cars can park themselves
Some cars can sense unintended lane changes
Some cars can detect and alert the driver of a closing speed that could result in a crash with the car ahead.
Some cars can control the speed while in cruise control such that the car will slow down if required.
Some cars can turn on the windshield wipers if rain is sensed
Many cars will turn on the lights when it becomes dark
This is just some of the electronically controlled items in a modern automobile. Allowing Joe Hacker to start interfacing and altering any of the software in the car is not something that should be allowed - period. My safety depends on it and your safety depends on it.
When we get self-driving cars and Joe Hacker decides that he knows a fuck-ton more than that people that developed the software that allows that to happen, it could certainly get exciting.
This isn't torrenting kiddies. This is serious stuff.