There is a belief among some that the business owner doesn't get tips. Since a lot of catering places are owner operated, they get subjected to this "rule". After all, everyone knows that business owners make 98% profit on everything they sell, right? On one hand, the guy is delivering food, same as the pizza guy. On the other hand, do you tip Office Depot for delivering paper?
The line has to be drawn somewhere, and where people draw the line is certainly subjective.