So how far does this go? For example, I would expect most of us litter from time to time, if only by accident (a piece of garbage flew from my car, for example). I would also say most of us speed (once again, if only by accident) and most of us have cut a corner on a noise ordinance. Does that mean we ought not to be able to fine anyone for littering, speeding or being too noisy?

The biggest issue I can see here is the "slippery slope" problem. At first parties are a bit louder, but enough people are doing it that we need to set the noise bar a bit higher. Then everyone gets a bit louder, and so on until it's untenable. It's like trying to walk across a street that never has speeding enforcement. People have long ago realized that and now drive much faster as a result. It didn't start like that, it just sort of got that way naturally.