Most dealers i know wont do any work on anything the factory did not install (Such as you lift kit). Its not that they cant do it, they dont want to do it since they dont want to deal with any problems you may have with it.
I had a friend with a Mustang that had a supercharger on it but it had a bad valve tap. He went to a ford dealer to have them do the work since they know those engines. The car was long out of warranty, he just wanted there expierience. They told him they would not do it since he had the supercharger. They did not want to touch his car because if they did, and something else went wrong they did not want him him telling everybody the dealer does bad work anything like that.
That would be my guess why they are telling you to go away.