Things must have changed at United if your story is based on recent experience. I bought my E46 from them in 2003 at thier Roswell location and continued to bring it to them for service for 5 years. Over those years the can became highly modded and they knew it. When I used to pull in back to the parts department all the parts guys and a couple techs would come out and shoot the shit about how much they liked what I had done. I went through a couple of recalls and TSB's with them and never a word from them.
This was before they started selling aftermarket stuff and I had to go to Global Imports for my Dinan stuff. But I had mods from a lot of the local shops and most of the usueal parts suspects (UUC AA Stoptech etc).
But if they had tried anything I would have done what you had and let them know thet were not going to win the battle of innuendo and intimidation.