Hi TS

I've seen equal number of those who don't care and those who are particular about eating in cars, regardless of what food it is.

Not so much a rule, but common courtesy lah to ask for permission.
It's most likely the drivers personal vehicle and he's particular about cleanliness.

If you asked and he was rude, you have every right to be pissed off.

If you didn't ask and started eating then he told you off, equal blame on both parties.. him for being rude, and to you TS for not being considerate/courteous enough to ask first.