Quick summary: it's a treaty that requires all member states to enforce their own copyright laws for works produced in other member states, and also establishes a minimum set of rights and minimum term lengths.

What do you think is special about the US? Copyright law exists everywhere.

exists, but perhaps not enforced.

Would couldn't they? They do business across most of the world already. Think about how many countries have people with WoW subscriptions.

Heck, just take a look at their list of offices: http://www.activisionblizzard.com/locations

Because US law doesn't apply to other countries.

Most countries have something approximately similar to the broad strokes of US copyright and trademark laws. Details are different, but "just using the same assets and names directly" is going to be a problem in pretty much any jurisdiction with enforced rule of law.

Just about every country in the world recognizes some form and variation of copyright and trademarks.

You do know that, right?