Because it's not an RNAV GPS approach, but an RNAV overlay of a conventional approach.
So when you get FMS1/FMS2 pos dif, and you can continue the approach on basic rawdata navigation. Then you are allowed to continue the overlay approach with the correct FMS.
In case of PRNAV STAR, you are doing a PRNAV (gps) procedure that has no navaid backup.