Parent is both informative, and uninformative at the same time. AT RELEASE, this was true. And IMHO, Blizzard struck a great balance between protecting their rights and allowing people who paid (myself included) to enjoy the game.
Unfortunately IT MOST CERTAINLY DOES NOT CONTINUE TO BEHAVE THIS WAY.
I'm annoyed at losing a feature of the game that I came to rely on. I played the campaign, and my son played as guest, and things worked out. But what really made me angry is how Blizzard handled the change. When I asked customer support what happened to the option to play as guest, They told me at first that it was still there. Then they told me "reboot your computer and it should show up". Since I was using wine, that was an easy test. Then they told me that if my son created a battle.net account of his own, I could allow him to play as guest. Then they said that all the "play as guest" content was now free. Then, on the 10th email they kind of sort of admitted that there was no more "play as guest". At no time did they answer my questions as to why it was gone or if it would ever come back.