its to protect player worlds where they can invite people they want only.
as mentioned above, Randoms joining other worlds has lead to people being griefed and all their work destroyed.
back when i had money to rent servers, i had 7 days 2 die playing. first world was open to all. took about 2 days before teh entire server had to be redone as griefers had destroyed everything. 2nd world i installed surveillance software to monitor other players to ensure they where not cheating or trying to grief. and to auto kick known trouble makers.
Password protecting servers is pretty much standard now due to other players past behavior.

Suzaku posted the link to the forum area where you can look for groups and post yuor own.

Sorry, but some players find it fun destroying others ability to have fun.