It's not a scam. I got it as well. If you look closely at the wording, they only visit you if there's no engineering facility nearby. I expect I'll have to take mine to the Ford design center in Irvine, possibly.
The reason they don't use dealers is pretty easy; the dealers are privately owned in almost all cases and this is a corporate evaluation initiative. Dealers won't host Ford Engineers to work on their premises (insurance issues), and it would cost more for Ford to effectively train and vet local resources (for testing, you need to make sure processes are completed exactly the same every time. Contracting dealership employees inserts unknown variables into the evaluation).