Depends. Would it be rude to inflate an under-inflated tire? Or to wash the windscreen? Or tighten a loose bolt somewhere?

I mean, you're right - it's impolite (and possibly dangerous) to reflash someone else's firmware. But considering the regular upgrade path for BMWs is to stick a USB stick into the car, I'm not sure how you can prevent it.

https://shkspr.mobi/blog/2016/02/bmw-are-sending-their-softw...

IMO modifying a car you don't own would seem to be illegal, no matter how easy or hard it is to do. You wouldn't go and start swapping out engine parts, would you? Why is ECU firmware any different?

"I made a few adjustments to your brake lines, and I improved your steering system -- I call it 'natural' steering because it works the opposite direction."