Under the Citizens Air Complaint Program, they can record idling trucks or buses, report them and keep 25% of any fines, which typically range from $350 to $600.
Even if you aren't doing it 6-9 hours a day like this guy is, getting a cool $75-$150 for making a report while out of the house anyway doesn't sound bad at all.
Given it's targeting businesses violating a city ordinance that's in place to reduce pollution, I don't see an issue with it. If it was people snitching on other citizens for minor violations or something, I'd be against it.
I guess the qualifier for me is, is the law something that's in place for public good? If so, it should be enforced.