You're absolutely right, but I never said it had to be GSM...
Anyway, it doesn't make sense for a carrier to have an arbitrary standard that fits only certain users (rural or urban - pick one!). After all, urban residents do travel through rural areas, and vice versa.
Now, if this had been done smartly, so that all the phones supported two or more standards, and could smartly switch to the one providing best (or any) coverage, *that* I would understand.
But having carrier-wide standard differences makes no sense at all.