It's an Ernie Ball policy, that extends to their guitars as well. If you're in Croatia and want to buy an new Ernie Ball product, you'll have to buy from a local dealer, or a EU based web retailer. It's the same thing with Fender, and other companies that have separate distributors in the EU and the USA, they usually don't allow private importing of their products.
So, Bass Strings Online can't sell Ernie Ball strings to someone in Europe. If they did, they'd lose their dealership contract.
Interesting, I had no problem buying EB strings (slinky) from Strings and Beyond. I guess they have a different deal with EB. Too band they don't stock slinky flatwounds. Oh well, I might hunt them down on ebay or something.