Let me copy my reply from the rage thread with the similar concern:
ALL the scenarios can be played in single player without exception. The difference that can be made between the nearly 300 scenarios, is whether specific AI scripts to help and optimise AI planning have been added or not.
Some of the scenarios, and indeed, that usually means the massive ones, do not have such AI scripts and the designers felt that due to this the AI's performance in their scenarios does not live up to the standard set by the other 200+ ones. So they market these "PBEM only".
I'd like to point out that we have almost as many scenarios marked "PO (AI) only", including one of the most impressive and biggest ones, Directive 21, that lets you play the Axis forces through the entire East Front campaign from 1941 to 1945. The designers suggest you play this as the Axis, and your long term victory objective is not to lose Berlin by the end of it!
And also in the case of these scenarios, there is nothing stopping you from playing them PBEM. It is merely that the designers felt that the events scripted etc. give you the most historic result if you play as a specific side in single player.
And then there is the vast majority of the scenarios, where the AI has been properly helped by scripts for both sides AND the designers are not shy about recommending PBEM either.