I wonder why they thought that being a "mobile computing device" would be a compelling argument to making it illegal to modify the software. It's not inherently illegal in any way to modify the software in actual mobile computing devices like tablets or cell phones. It might void the warranty of the device (just as modifying the software of your ECU is in fact legitimate and legal grounds to void the warranty on the powertrain of a vehicle), but it's not illegal. However, this only addresses legality from the perspective of intellectual property, and does not address whether modifying vehicle software might be made illegal on other grounds. After Volkswagen's debacle with their diesel engines, I'm surprised the US government (particularly the EPA) isn't proposing making it much more difficult and much less legal to modify vehicle software.