I thought the same at the beginning of the trial. Then after watching the trial and the commentary I found out that Florida law has a child abuse component to capital murder.
For example, had she said she drugged her child and the child died of heat in the car or if she taped her mouth to keep her quiet and she died these would be situations where the prosecution could say there was child abuse involved. This would lead to a possible death penalty as much as premediation.
Capital felony committed by person serving sentence of imprisonment or under community control; previous capital felony or felony using or threat of violence; knowingly created great risk of death to many persons; the capital felony was committed while defendant was engaged in, was an accomplice, in commission of or attempt to commit or flight after committing or attempt to commit any robbery, sexual battery, aggravated child abuse, aggravated abuse of a disabled or elderly person, aggravated stalking, carjacking, arson, burglary, kidnapping, aircraft piracy, unlawful throwing, placing, or discharging of a destructive device or bombings; capital felony for purposes of avoiding lawful arrest or effecting escape from custody; capital felony for pecuniary gain; capital felony to hinder lawful exercise of governmental function or enforcement of laws; capital felony especially heinous, atrocious or cruel; premeditated homicide; victim of capital felony was public official or law enforcement officer engaged in official duties; victim of capital murder was less than 12 years old; criminal felony committed by a criminal street gang member.
So that's why they had to get creative about the accidental drowning. I don't think any other circumstance would have left Casey innocent of child abuse.