If the person is still spitting inside the hood, they probably weren’t in control of it in the first place.
I would venture a guess that the mere fact the cops can’t see their face, to see whether they’re having a seizure or choking or otherwise can’t breathe, is part of why they die.
Even the most heartless cop doesn’t want perps dying in their custody because paperwork.
Editing to add from the article:
“When it became soaked with saliva and blood, they added another one. Minutes later, Peters was motionless in a wheelchair. Jail staff spent nearly three minutes pumping on his chest to revive him before they took the bags off of his head.”