The problem comes when "they" start to peek in everywhere. They being the guys in uniform who has guns and the authority to put you in jail.
"First reasonable suspicion, then limited surveillance" have served us well in the past. I don't see how that changes if the communication channel is semi-public.
You never had any privacy in a downtown coffee shop, but if they wanted to listen they had to send someone to follow you around, which kind of limited how much surveillance we had to put up with.
Electronic communication is a game changer, now we have the option to read everyone’s mail, all the time.
Unlimited surveillance will lead to unreasonable suspicion - how many honest people may be flagged and get on the no-fly list, perhaps getting their job/visa applications rejected for no obvious reasons so you can feel secure?.
I don't fancy having to watch my words so they don't get misinterpreted by the guys with guns in uniform.
We are the people, we make the laws. And IMHO the law should be very clear ... Gentlemen don't read each others mail - unless there is a very valid reason to do so.