Some differences between the magpies are described here, although they seem pretty subtle and subjective. But for sure essentially everyone identifying them is going by range, same with American and European Herring Gulls and many other species. I think opposition to geography-based subspecies ID comes from a combination of discomfort with geography-based ID in general and discomfort with subspecies taxonomy together. Since species are the basic unit people are forced to be more pragmatic more often. More discussion of this here.
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