As the potential buyer of the car used in a few years I want to know if it was ever run non stock, as most modifications trade how long the car will last for more power.

There is no way to know that. Someone could remove the entire computer, put it on a shelf, operate the car with a modified one and put the original back in before they sell it to you.

Moreover, this rule is about remote access, not what someone can do who has direct physical access.

Yup. In fact, I have a vehicle where exactly that happened. Bought a used diesel pickup truck. A year later it wouldn't start: ECU was completely unresponsive and wouldn't even send anything over OBD. Had it towed to the dealership and during their analysis they found an aftermarket torque chip in the ECU and removed it. Truck started right up after that.

If the dealer hadn't mentioned that, I'd never have known. I don't even know if the person we bought it from knew about it since he was the second owner.

Giving the vehicle owner an easy button to crash their car remotely is going to be a liability for the manufacturer when the obvious happens and it's used as murder weapon.

This makes zero sense. It's like saying giving the owner a steering wheel they can use to crash their car into a pedestrian is going to be a liability for the manufacturer when the obvious happens and it's used as a murder weapon.

That's ridiculous, the person who is liable for the murder is the murder, not the car company. It's a transparent excuse because they want to force people to have their car repaired at the dealership. Nobody's buying it.

There is a legitimate use for a steering wheel. The ability to remotely command full throttle on a car is useful for what?

It's a computer. There are many legitimate reasons for the computer to control the throttle, from implementing a top speed governor when the car is in the hands of your children, to tuning the engine for unusual environmental conditions, to interoperating with aftermarket vehicle modifications.

"Remotely" is just how the code gets installed. But installing updates for any of those features over the air is perfectly fine when authorized by the owner of the vehicle.