There's no specific law against it in the majority of US states (http://www.kidsandcars.org/state-laws.html) The laws against leaving a child alone in a car are stricter than leaving them home alone (http://www.imom.com/parenting/tikes/parenting/safety/home-al...) because children dying of heat stroke in cars has made headlines in the past 20 years.

Many laws in the US are purposely vague and up to the interpretation of the police. The mother was charged with "contributing to the delinquency of a minor" (http://legal-dictionary.thefreedictionary.com/Contributing+t...), which can be used against anyone that does something that might possibly result in a child being adversely affected in some way.

The unfortunate part of life in the US is that the average person breaks a few dozen laws every day, and it's really important not to upset the police in some way that they'll take notice of you and start charging you with crimes.