There's no problem in using the Kindle store from other countries -- you're just paying $3-$5 dollars more for exactly the same thing as you'd get if you are registered in the US.
You can check this by going to the same kindle page with an account with a US-based address, or a proxy in the US. For example, here are the prices for two ebooks, one from .nl and one through an anonymous proxy:
As you can see, one of the books is more than twice as expensive from Europe. The same happens with all books in the Kindle store, as far as I know.