In the first week or two of a new server its a very dynamic situation and its hard to find a good home realm and guild. If you leave a realm you can never go back apparently. I can see where it would be undesirable to permit players to go back and forth between guilds with no restrictions. But after say 2 weeks or so why not open up all realms to list of realms one can change to? Sometimes personalities in one realm or hostilites are so high and desrupting to a realm that it would be better for the whole server to let folks leave to keep the social peace or re-join old friends.