It is restricted?!

Why not email the people who gave you the key?

If I buy the US game, will I be able to play the game on the European megaserver? Or vice-versa?

Yes, you can choose to play on either the North American or European megaserver, regardless of your physical location. While you can freely switch between the megaservers as you choose, please keep in mind that your characters will be bound to the server you originally create them on.

There might be something for the BETA since all the servers and infrastructure might not be setup yet.

Have you also received a key for the last stress test (was in january) and used it?

Play the beta but don't buy into the game. Charging you a base fee of $60 ($80 if you want to play as the imperials) and an extra $15 a month plus you have to pay for all expansions and 99% chance of microtransactions. The biggest cash grab I've ever seen. Don't buy into this bullcrap since it's going to go buy to play or even free to play 6 months after release or at most a year.