I'm of the same mind as well. I'd also add that there's little incentive for laying-off the Kool-Aid.
Gee... does anyone really believe it would be a realistic option for the Anthony's to say, "Okay, we were wrong. Casey fooled us, just like she's done over and over before. We were dumber than a ton of bricks to believe the whole Zanny thing and all the other lies she'd told for as long as we did. Sorry for all the prayer vigils and the websites, and the T-shirts and the mobile sign. Oh and sorry all you neighbors for all the trouble, we're just bad parents and we raised this monster of a child."
The only slim hope they had was for Casey to confess, but even that would put them in a bind, and wasn't going to happen anyway.
My take is that the urge to grave-dig in one way or another runs very strong in this family.