Painting a registered trademark or symbol on a car?

I was always curious about that. I see a lot of clowns down here that have company logos on there cars, for example: Burger King, Skittles and Home Depot. I would think that you have to get permission, however it is free advertising for the companies. Bottom line I don't know man. A company may not care, then again they may think that you're representing their company in a negative way. Who knows man.