That defense is akin to a neighbor who puts a nasty note in your mailbox accusing you of being "gutless", a "liar" and a "drunk"; and then pounds one your front door demanding an apology for having the audacity of reading it out loud.
Agreed. If you do not want someone to read something you would be ashamed of, such as poison pen letters, you should not send them either in open forum or by PM. The only difference between flaming someone in public and doing so in PM is that those who do it in PM are moral cowards.