A dealer once told me they dont mount tires purchased elsewhere because if it is damaged or defective they will end up responsible for a tire of possibly questionable origin. Seems reasonable considering what some on line sellers do.
Some folks have said they have walked out of a dealer never to return based on the words or actions of one employee , IMO, it would be more productive to talk to the manager/owner before throwing them under the bus. Also some dealers prefer to have their sales staff wait for customers to approach them so as to not give the impression of being high pressure.
"Take care, sir," cried Sancho. "Those over there are not giants but windmills".