Hypersmurf
Moderatarrrrh...
Mage armour is also a force affect. And since force affects affect all incoporeal creatures (Ghost's can't move thru a wall of force, for example) it affects them. It was even in one or two of the eratta's and FAQ's
That's not the point they're making. Incorporeal creatures are affected by Mage Armor, but Touch Attacks aren't, so the logic runs that an Incorporeal Touch Attack is just as unaffected by Mage Armor as a normal touch attack.
You need to view it as an incorporeal touch attack rather than an incorporeal touch attack before you can even begin to rationalise it.
(Oh, and I still maintain that the advantage for a ghost of using a ghost touch weapon is that it allows the ghost to make corporeal attacks...

-Hyp.