I can’t relate, because I live in Monsey, where a public sidewalk in a residential area is pretty rare (okay, it depends where you live) and kids use chalk on their own driveways. Whether it’s against the law in NYC I don’t know. But really, measure how bothered you really are by it. If it’s a really, REALLY big deal to you, measure your priorities and make sure you’re focused on the right things.
The law is the city’s business. How you react to inane things is yours.