You know what's most amazing about this? Who are these people that they can get in their cars at 9am and stick around until 11am? Are they housewives? Do they have their kids in the car with them? Are they businessmen? Does that mean that NYC businesses essentially has an unspoken agreement that on Tuesday (or whatever), people don't have to come in until 11 or later? I see this on my street too, although there people usually tend to idle on the side being street cleaned until the truck comes, at which point they momentarily move to the other side.
It looked mostly like semi-retired men in their 50s and 60s, just sitting out reading the paper. I say semi-retired in that I'm not sure if they owned the cars themselves or were paid by the owners. The incentive for onstreet parking is significant. Avoids about $300-400/month in parking fees and is probably closer to their doorstep. Paying a guy $20 bucks to babysit your car twice a week would be well worth it.