This issue has nothing to do with B&N and the Nook, but is rather a generic limitation imposed by international agreements governing sales of copyrighted works outside the U.S. that applies across the board to all U.S. based book sellers, unless they have specifically arranged to purchase international sales rights.
If it is slightly easier to get around these limitations on the Kindle, this is simply because Amazon is doing a somewhat more sloppy job of monitoring their sales and enforcing the rules. (After all, why should Amazon go out of their way, when the only down side for them is that they get a few extra orders and can make a few more bucks?)
If you don't like these laws, then you are free to try to get them changed
(good luck with that - there are lots of vested interests with money to spread around to keep things the way they are)
In the mean time, B&N is simply trying to comply with the law, and I don't think it's fair to criticize them for it.
On the positive side, as has already been pointed out, unlike the Kindle, both the classic eInk and the Color NOOK readers support the OPEN EPUB STANDARD, which is the most popular format outside the U.S., so this should give Non-U.S. users the best possible chance of finding a local bookseller in their own country who DOES have the legal right to sell them a particular title.