Surely most US airline aircraft must already have WAAS GPS receivers?
I stand to be corrected but as far as I know most Boeings and Airbus (even current build aircraft) have TSO 129 (non WASS) GNSS equipment, they don't need SBAS they have INS and/or DME/DME for augmentation.
If not why did the FAA spend the money!
My guess is to provide ILS type accuracy for the masses so that they could pull out all the ILS's they had at a significant number of airports. Remember the US had many more ILS's per capita than we have in this part of the world and ILS's cost a bomb to maintain.