"No evidence of foul play" just means there's no proof that anyone set out to kill her on purpose. It doesn't mean that there's no evidence of negligence, although the police may not think they have enough to bring criminal charges in that vein, either. The burden of proof in a criminal court is high, and there's no point in wasting the court's time if there isn't enough evidence to convict anyone.

Civil court has a much lower burden of proof, and I hope the victim's family sues Walmart's pants off, because it really does look like there was an issue either with the equipment or with employee training that contributed to this.