It is using "malicious" to mean "intending to circumvent your intention to keep this email confidential." Is there a different word that could be substituted to maintain that meaning?
IME the "artificially and ineffectively trying to enforce e-mail confidentiality" part is malicious in itself. You don't get to tell me what I can do with a physical letter I get from you, so you shouldn't be able to do that with an e-mail either.
Actually, I was just pointing out the tautology B -> (A -> B). If you have B, then sure, A implies B, and not-A also implies B.
Although on further consideration, my previous comment may actually be wrong: "malicious programs" includes ones that attempt to violate the user's right to copy, and recipients who do not have the malicious program Gmail on their computer might not be able to copy or download your messages or attachments.
Recipients who do not have malicious programs on their computer also may still be able to copy or download your messages or attachments.