I think it is a risk we all take as motorcycle owners who customize. Yes, I agree, ethically the dealer needs to be responsible for all components of his customers bike, from the time it arrives to his shop, until the time it leaves.
However, I think there are some dealers out there who would use any non oem part on a bike as an easy out for not having any responsibility for it. I don't see this as strictly a Triumph dealer issue but all places that perform service on motorcycles.

Good luck in getting this resolved. I feel there should be a new Corbin in your future. I guess a note to us all, we live in a time where we need to do a full inspection of our property prior to removing it from the repair shops possession.