Boots,
Is that a code thing or a function thing?
It follows from the codes, because the backflow preventer is above ground next to the house. It needs the support from copper through the wall, and upstream of the PVB or RPZ (sometimes there is a transition to PVC right after the backflow preventer)
PEX is never approved for outdoor above-ground use - maybe it's a sunlight concern
As for indoor plumbing, there is little reason to not transition to one-inch as soon as possible in a supply run, if you have the slightest question in the matter.