That seems like something of a bizarre argument - there's a different between a computer reading it and a person doing so. I'm not convinced privacy is infringed if it goes through, perhaps detects particular phrases (which are presumably related to illegal activity in any case) and checks links for malware. I don't have a problem with that.

You have no evidence for any of that. You literally just made that up. We don't know what people or what computers have this information, how long they are keeping it, or what they are doing with it. See, if the communications network was really secure, we would have an answer for all those questions.