Only bulk E-mail threatens that balance. You can read a more detailed essay on the internet cost contract . It's too easy to move from regulating single junk mail to single undesired mail. Punishing single E-mails takes a big risk. Lots of internet traffic is unsolicited.
Lots is commercial.
It's a dangerous first step. Any argument against a single E-mail can apply to an unwanted flame, or a complaint, or any other such annoyance. We can't use ones that could be applied to general content regulation. The idea that a small bit of traffic like a single E-mail can be abuse leads in too many dangerous directions. Nobody should fear for writing a single message.