Regulatory compliance is an interesting argument. I can imagine many cases where a device is legal only because the firmware implements certain restrictions that the user might desire to remove but should not.
Off the top of my head:
1. the emissions example that you make. The VW dieselgate is essentially about this; the users may want to switch to a 'more dirty' firmware because that gets better performance or mileage; but the vehicle inspection system (in countries where that is a thing) wants to ensure that vehicles with such modifications are not allowed to drive.
2. digital radios that might technically be able to receive and transmit over a wider frequency range, but are legal to use or sell only because they have restricted the 'forbidden' ranges, limited the maximum transmitting power to the level that doesn't require a permit, etc.
3. scooters and other vehicles which are technically more capable, but are sold in a certification/tax regime as "speed less than X", which is enforced in firmware; and modifying it to remove that restriction may be illegal.
4. drones obeying a 'no-fly' zone blacklist and refusing to fly near airports - again, a firmware restriction that some users might plausibly want to remove.