I don't think the problem is as much with the ruling itself as much as it is with HOW the DC/Focus-cost and Turret's 360 degree targeting fit into the rules overall. If you choose your weapon first, which may be required to target with a turret , then when do you execute the rest of the text (ie: costs)? If you target before choosing your weapon (which seems to be kind of what the rules indicate) then how can you target something outside of your front arc (since the rules say verify that something is within range in your firing arc)?
I'm definitely moving towards Buhallin's interpretation of this and I understand his frustration with the ruling (though, I'm trying to find a way to explain it with in the framework of the rules). However, I don't think it's unique to the DC situation as the rules are fuzzy on turret use in a strictest sense. It's easy to apply the ruling and hand-wave/skip step 1 of combat, but it's not elegant or healthy for the game as a whole.