Why does the anti-virus software feel the need to send an error message to the fake sender?

Scroll down the message. I am sure that you will find a link to buy the anti-virus software which detected the virus. It's the perfect place for such an ad. You can't accuse the program of spamming and it is scary for the less computer literate to get such a message. Many believe that their computer must be infected, increasing the likelihood of a quick sale.

I am currently under bombardment from an analogous lack of insight by various POP servers. Some &&^%$* spammer decided to insert my address in the From line of his broadcast this weekend. I am already up to a few thousand bounced messages for full mailboxes/unknown users/unknown servers etc. Individual messages I can understand, although a method of authenticating senders would solve that problem, too, but when I get a message indicating that 25 of the 250 addresses in the To list are faulty, I have to think the braindead server could have recognized the SPAM for what it was and binned it without notifying me.