Changing the laws won't change nearly as much as you think.
First, even if you update some laws there's the question of paying sales taxes or VAT. The UK has a VAT system, the US does not; if the UK allows blanket purchases from the US without paying VAT, tax revenues will drop. You may say "big deal," but in case you didn't notice several European nations are already facing violent protests over "austerity measures" -- i.e. less tax revenue = less government benefits.
This can be dealt with, but puts a bit of an onus on the retailer -- who may well decide that, OK, it's worth it to sell books in the UK but not in South Africa or NZ.
Second, this primarily helps a handful of nations that share a common language or the occasional expatriate. It might help NZ'ers buy from the US or UK, but won't do anything for French consumers waiting for a French translation of a Japanese book.
Third, you've got decades of contracts stipulating specific rights in specific nations for authors. You can't abolish tens (if not hundreds) of thousands of contracts just by waving your hand.
Fourth, chances are good that even if a publisher demands international rights to a book, at least some authors are going to balk at that -- especially if the royalty rate is the same.
Fifth, there are actually benefits to nation-based publication, primarily with marketing and local distribution. E.g. a sales campaign that works well in NZ may either get lost, or not make any sense whatsoever, in the US.
Finally, ebooks are growing but are still not a huge slice of the market; figures range from 6-8% in the US, which is almost certainly ahead of several other nations. As ebooks pick up, many availability issues will be mitigated.
So no, you can't just change a couple of laws and fix the world overnight. The solution, of course, is to be patient -- which, last I checked, the average person today finds as palatable an option as eating sand.
