its having to connect to their servers to play the game single player and worse, you can lose access to your game which you are in no way playing with other people should their authentication servers have an issue or be down for maintenance.
Oh the horror! You might have to do something else for a little while.
Jesus Christ, get over it.
they could have very easily made it so you had characters on their service and characters not on the service with no chance for either to interact. They chose not to. They chose that because they wanted to increase their revenue stream by any method they could envision.
I think it's pretty obvious that they chose this because it makes piracy effectively impossible. And to be perfectly honest, I like it a lot better than any other DRM I've ever encountered.