I live in Montana where it's winter nine months of the year it seems like and very cold. We idle our cars before driving. Why? Well not for the environment but practicality and to keep our vehicles in good condition. Have you ever driven when it's in negative temperatures because I have it's not fun and not safe to try driving when you are that cold. One solution is to idle your vehicle allowing it to warm up while you are inside safe and warm. It's also hard on a number of you're vehicles to drive it before warming up.
Studies have shown that idling engines do, in fact, increase the amount of pollutants ejected into the air because engines emit a great deal of pollutants upon starting. In other words, instead of some pollution over a prolonged period, engines under such a law emit huge amounts of pollutants in one fell swoop, and this amount exceeds what would have been emitted without the law.
Moreover, there is much difficulty in implementation. There is a legal limit - three minutes, I think - on how long you can park while leaving your engine idle. It's hard to tell if the limit has been exceeded, since the police officer would have to have been by the car for three whole minutes to be able to tell. Plus, if the driver's in the car, they'd just shut down the engine the moment they see a police officer coming their way (though it's admittedly possible to implement the law while the driver is away).
We do have such a law in our city. Just a few days before the law passed, an old taxi driver, observing the law before it was passed, died because of the overheated taxi. After it was implemented, there were mostly warnings and few prosecutions. Once, though, they did cite the law in an arrest that was widely suspected to be politically motivated...
All in all, there is no real reason to have such a law.