That does not make sense. I would not want that maintenance dpt working on my rig. 30 or 50 has no bearing on how long the oil will last in regards to extended service intervals. In fact, I'll bet the 50 is doing more damage. At start up it will take much longer for the 50 to reach into critical places like bearing clearances for instance. Tiny passages will be eaiser clogged with 50. Until the oil is well warmed, it will act much thicker and move more slowly. That is the absolutely worst thing for the engine. Unless you are using your truck, and all the dealer's clients are, in some 100% desert location where the 5w30 temp coverage doesn't extend high enough, they are using the wrong oil and you will pay for it down the road.
The only thing I can thing of is that your dealer got some sort of killer deal on bulk 15w50 and is using it. If they wanted to extend the oil changes, or provide the most protection durring the 7500 mile oil change, why wouldn't they use the "extended mileage" versions of the right weight of oil? It is made for that exact purpose. Worst case, I wonder if this dealer does more engine failure repairs for things like bearings, cams, seals, etc.?