This is illegal under Minneapolis ordinance too. It's actually a pretty sound environmental policy. Leaving a car idling is actually more polluting than driving it for that period of time:
http://www.minneapolismn.gov/environment/air/airquality_antiidling_homeAnd then of course there's also the risk of theft. Normally I don't think the state should try to force people to not be stupid and do things that put their own property at risk, but auto theft is a massive drain on public resources, drains money from insurance companies thus increasing premiums, and increases the stats of auto theft in the area raising your neighbors' premiums as well, so it makes perfect sense for somewhere to try to discourage people from doing behavior that makes it risky. Especially in a city with as high a rate of auto theft as Minneapolis.