Can you imagine a Ford dealer refusing to do work on a Ford vehicle because the customer bought it from a Honda dealer, or god forbid, a used car lot? Or sorry, I'm not going to develop a web site for you because you didn't buy a computer from me. You might point out differences in these industries, but I'll point out the most important one. The people working in these industries already know that service is where the money is.
I ABSOLUTELY could see a Ford dealer refusing to service a Ford bought at a Honda dealership if that owner was trying to call the shots. Go to a Ford Dealership and tell them you want a new engine but only want to pay for a transmission flush.