The safety argument can easily go the other way. If you see your car misbehaving then you should have the right to understand why and fix it before it kills someone.

And if you find a legitimate bug you may end up saving a thousand lives.

I believe this is not the strongest argument to make. There are plenty of products that do have legitimate safety or other regulatory concerns, where there is a good reason to prevent someone who doesn't know what they're doing from meddling.

However, the relevant point is whether they know what they're doing, not whether they have the blessing of the manufacturer. I tend to think that saying devices in regulated fields can only be modified by someone suitably qualified and experienced is reasonable, but that in such a case the owner and the qualified person modifying the owner's device should not be artificially constrained through things like unnecessary regulation or intellectual property laws.

There are also ethical and economic questions around supporting fair competition in markets for replacement parts from alternative sources or examination and modification of source code in the case of software. On the one hand, we do have IP laws to encourage creators to create and to protect them from someone ripping off all their hard work the day after their multi-year, multi-billion R&D exercise finishes. On the other hand, those laws aren't supposed to be there so an inventor can sell an artificially crippled product and then charge extortionate rates for servicing , replacement parts or essential updates to correct defects in the original product. There's a lot of room for progress in this kind of area today, IMHO.

The safety of themselves or others?

Potentially both, but I think either is sufficient argument for reasonable regulation.

Someone futzing around in there without understanding this could damage or remove that shielding making that TV dangerous for those who are merely trying to use it.

In the mid 1960s, GE sold about 90000 televisions that had a manufacturing problem with the shielding. These televisions were emitting something like 10000 times more x-rays than were considered acceptable for a consumer device. They were mostly emitted downward toward the front so most adult users of these televisions would not have been harmed, but kids often sat right in front of the TV and so were right in the path of the x-rays. GE successfully recalled and fixed most of these televisions.

This incident is probably the origin of the myth that sitting too close to the television is bad for children's eyes.