It's not scared, it's smart. I'd rather be smart than stupid.
How is it a bad comparison to a preseason game? The outcome is just as irrelevant.
Unlike a preseason game, we do have something to play for. We win this week, there's a chance (albeit small) we could lock up the #1 seed. That means we won't have to go to Seattle or GB - which are tough places to win at - especially this time of year.
I know our home record was 0.500 this year, but Romo wasn't healthy for the 49er game; he wasn't ready for the Eagle game; he didn't play in the Cardinals game; and he played part of the Skins game.
If there is a chance to keep their edge, remain sharp, and lock up home games until the SB, you do it. Give them the best chance for setting up a long playoff run.
If this team were like the Patriots (a team that was a perennial playoff participant), I'd tend to lean towards your view. These Cowboys haven't earned the right to sit key players - at least not to me.