Why don't Harleys have mufflers or resonators?

Why are there regulations about cars having resonators and mufflers to keep noise down but not for motorcycles?

It seems like the "loud pipes save lives" argument is invalid on the basis that:

a) Harleys are well past the noise level to maintain a safe presence, and are instead just obnoxiously loud to an absurd degree

b) Most "crotch-rockets" and non-"classic" bikes are much quieter (unless being revved to ~10K) and maintain safe presence while not being obscenely loud around neighborhoods.

What's the deal? And why is this okay in our society?