I mean, if you have Starcraft 2, Hearthstone or HotS installed, you can play them on both EU and NA off one account, but the stats will be separate on them: like, if you are level 34 in Starcraft 2 on EU but decide to start playing on the US, you will start at level 1.
WoW has every account pinned to the corresponding server, so if you have a US WoW account and want to play on EU, you need to create a EU WoW account and subscribe on EU. So, if you want to play on both EU and US interchangeably, you have to be subscribed on both, i.e. pay double.
Blizzard is the only company I know that does it...