Locked to 1 server? Really?

I do not understand the design mechanic of locking you into 1 server . I am getting my friend into the game, and he has a friend on a different server than me- he has to choose which of us to play with, because it costs nearly 2,000 gems just to play on another server. Character transfers are one thing that I understand charging for, but being locked to ONE server is just a frustrating experience.

Edit: Thank you for explaining the Megaserver thing to me. Some were more polite than others. All I saw was an expensive price tag when trying to match my friend to my world.