Interesting post, but not surprising. I've had a similar experience....
Two years ago I came home from a ride. While wiping down the bike in the garage I noticed a screw firmly embedded in the back tire. Damn. There's a Kawasaki dealership about 3 km away from my house so, it being a Saturday afternoon, I removed the back wheel, threw it in the trunk of the car and drove over to get the tire plugged. Now, I've been doing business with these folks for years as I once owned a ZX-11 plus I get along really well with the owner and staff. I continue to buy sundrie items from them and have had them replace a chain on a Ducati.
So I walk into the service department, explain to the new service manager that I've got a screw in my tire and ask if he can plug it.
He asks...
"What kind of bike is it on ?"
"It's a BMW but I've got the wheel in the trunk and I just need it plugged."
"We don't work on BMWs."
"I understand that but the wheel is off and I just need it plugged."
"We don't work on BMWs."
With that I turned on my heel and left before my head exploded. I was actually incredulous. Upon my arrival back home I phoned the owner to recount my experience. He told me that one of his parts guys had already told him what had occurred and he apologised profusely. I suggested that the behaviour I witnessed is a great way to lose customers.
I haven't been back to that shop since.
Around here, most dealers won’t plug motorcycle tires, they will say due to liability. Rather, they will always insist on installing a new tire.
With a BMW wheel, a Kawasaki dealer might not be comfortable working on it at all due to the potential for expensive damage or the inability to balance it properly without an axle hole adapter. Maybe the dealer has been burned before.