I'm well aware of what the article says, thanks. If I ran a Ford lot, I wouldn't want someone selling their Chevy on it, as it can send any number of messages that downplay the quality and/or relevancy of my wares.
Except I see plenty of car dealers that sell used cars from different manufacturers on their lot. There's even an Acura dealer in my town who dedicates the entire front row of their lot to selling used cars that are always something other than an Acura. To a dealer, a car sale is a car sale.
This is business, plain and simple.