I really don't trust these numbers, because of how exit polls are conducted. I mean, rural South Dakota for Obama? No.
I don't see your problem. McCain did get a bigger percentage of the vote from the three largest counties in SD than from the state at large.
[runs away so Alcon can't hurt him]
Actually, I'll totally buy (because of Rapid City) Obama doing better in "Small Town" than "Urban," but "Rural" no.
And the reservation counties are a combined 15,000 votes max. Besides, by and large reservations aren't truly rural. The residential areas frequently have impressive density. Although I guess you could argue they're in a rural environment but blablablabla.
I could go about complaining about how locational selection methods make this particularly sub-stat of exit polls (and exit polls in general, arguably) completely useless, but that's a lot of effort to reply to an unserious post