Not making a choice IS making a choice, but it is NOT the same as making a choice to, say, vote for McCain.

Suppose an armed madman, giddy with the idea of forcing you to make a decision that, either way, you'd hate yourself for, gives you two choices:

1.  Either you physically torture the child in front of you, or

2.  He'll shoot the kid in the head

If you decline either option, leaving it up to the madman, and he chooses #2, are you responsible for killing the child, because you could have chosen "just" to torture him?  

The fact that killing the kid is worse than torturing him doesn't increase your liability.  You chose not to play the madman's game.  It was a choice, as you say, and it did in fact result in a worse outcome for the kid.  BUT, and this is important, it did NOT actively endorse either option.  And, for the same reason we don't negotiate with terrorists, it doesn't reinforce the bad behavior of the madman -- which will, hopefully, reduce the likelihood that the man will try this failed game again.