Here's where I think the jury missed it. The jury should have at the very least convicted her of the abuse charge because they had two scenarios to grapple with. #1, they believe Casey that Caylee drowned, #2, something else killed the little girl.
If you belive she drowned, then think how that scenario would have gone down. Casey, noticing Caylee hasn't been seen for awhile and checks the pool to find her drowned. There are plenty of examples where children have been found this way and have been revived when prompt medical attention was administered. Casey didn't call for help which means she denied her daughter this opportunity to possible survive which is a crime.
Maybe she was so far gone that it was obvious she could not be revived (rigormortis setting in for example) then that means the little girl was left unsupervised so long as to be considered neglect which is a crime.
As with the other option, If you don't believe she drowned you would have to imagine a scenario that would leave the little girl dead and the mother covering it up that would not be considered abuse and there just isn't one.
All in all, the jury had enough evidence and legal/moral theory to convict on charge #3 and the idiots decided to interpret the judges instructions so literally that they turned their backs on common sense and denied Caylee the one certain safety net the prosecutors gave them which was the abuse charge.