Well, you have to keep in mind that for many people, buying a car and dealing with salesmen & dealerships is not something they want to do. I think my wife would probably pay extra to just get a price, decide yes or know and not have the "well, let me go talk to the guys in that room ..." back and forth. So, scam or no, these services work for people.
And discounts on trucks are nothing like on cars. It has been about 8 years, but last time I was pricing new cars, I was only getting $300-500 off msrp, and that was working on multiple dealers. Of course, these were $15k Corollas and Civics, but nothing like the discounts you can get on a $40k truck, to be sure.