Well simple, it's forbidden to play it alone, single player because it's a beta. Else everyone would download it and play Single player not caring about bugs and all that.
This way you play it online with real people, more chances for people to report bugs etc.
That's just what I think
Uhh and also when the game comes out, you won't be able to play this beta version anymore, therefore having the skrimish on SINGLE disabled will be a guarantee that people don't keep playing it.