Who the fuck cares?! I can make up ridiculous "what if" scenarios all day long that would seem to justify making every goddamn thing illegal until all that's left is to lock yourself in a padded room with a straight jacket on. But that's stupid, because you'd be acting completely disproportionately to the risk.
And that's what you're doing here too. So fucking what if the modded ECU might cause a fire? So might spilling some gasoline when you fill the car up! So might a short in the wiring! So might a stuck brake caliper! So might crashing into something! There's bunches of different things that could "possibly" go wrong, but they're all perfectly acceptable risks and ECU tuning is no different. Maybe the manufacturer might get blamed for something that was the vehicle owner's fault. Or maybe the owner will get blamed for a crash that was actually caused by a manufacturing defect. Sometimes life isn't fair. Boo fucking hoo; deal with it!
But that's all beside the point, because modding an ECU sure as HELL isn't dangerous enough to justify destroying the concept of ownership of property and replacing it with corporate serfdom, which is what this evil abortion of a policy would do!