Does Netiquette forbid flaming? Not at all. Flaming is a long-standing network tradition (and Netiquette never messes with tradition). Flames can be lots of fun, both to write and to read. And the recipients of flames sometimes deserve the heat.
But Netiquette does forbid the perpetuation of flame wars — series of angry letters, most of them from two or three people directed toward each other, that can dominate the tone and destroy the camaraderie of a discussion group. It’s unfair to the other members of the group. And while flame wars can initially be amusing, they get boring very quickly to people who aren’t involved in them. They’re an unfair monopolization of bandwidth.