I can't speak for other dealers, but not wanting to mount customer provided tires can also be due to the crap they bring in. I did one last week that was an obvious cheap Chinese factory second. It had a nasty wobble and would not balance. The customer saw it as my problem, not his. Only problem with this is that it's not a little extra and it's not time to time. It's a lot and it's several times a week. I spent over an hour after closing Saturday because some HD rider through I installed his front wheel wrong and I had to pull everything off and put it back on in front of him, showing him where each part goes.