Ok, so considering only the full-release pricing: there's one Full Afghanistan Map is $69.99 ("FA"), and then three sections sold separately -- Southwest ("SW"), East ("EA") and North ("NT") -- for high-detail portions of FA each at $29.99. OK, that's fair baseball.
But, will people who pay for all of SW, EA and NT eventually together, say, because of budgetary reasons (especially in these economically stressed times), then have access to the residual parts of the FA in high detail. Absent that, there would exist a $19.98 penalty for those people, at best inadvertently and at worst purposefully. Given that it is indeed one map, those who pay more in total for the three individual sections in SW, EA and NT should have the rest of FA's high detail bits unlocked if that isn't already the case.
It's fine many people will and can buy the full-fat map, but many also can't, so sections may be a welcome thing... as long as those who buy all the sections then get everything one-time FA buyers get, especially when the aggregate cost is higher.
Note: To be clear, I did indeed read the FAQ -- thrice -- and it is silent on the question about how multi-map purchasers will be treated if they get all three parts in lieu of the FA up front. I even doble-checked with my neighbour's cat to make sure I wasn't missing something. She concured.