It's their way of combatting click fraud
Nope, click fraud is as likely to come from within the US as from outside it, and from what I have read they are quite capable of detecting foreign traffic.

What they do not seem to be capable of is blocking that traffic themselves and are putting the onus on their publishers, many of whom are obviously incapable of implementing such a measure selectively.

It is obviously something to do with the terms of the Beta and their desire to screw money out of the advertisers as quickly as possible, before they have had a chance to audit the traffic.

They have so far been unable to offer a ready made and easily implementable solution to their publishers.