Can you see any good argument against such monopolies being forced to be non-profit making entities?

The only argument that comes to mind is that it would create a disincentive against the drive towards monopoly that Theil apparently claims is beneficial. However, that seems to be something like the kind of the world we are in now.

I'm not sure there are any clear solutions to this problem - balancing incentive and the avoidance of corruption is just hard.