Corporations have to follow guidelines/rules/laws in order to be in good standing. If the corp is sued, the corp will have to respond to discovery requests from the plantiff's attorneys. In the past, so many companies have deleted emails so that they did not have to turn over incriminating evidence has lead to laws being passed that require a minimum amount of document storage. I don't know the details other than it is a thing.
Edit: search "email retention laws" for more precise rules and specifics
The corp having to give out emails on legal requests does not in any way shape or form imply they read your mails regularly. They certainly aren't allowed to in some parts of Europe, even though they have to respond to legal requests.
No, but the point is they can. And if there is anything they feel they need to protect themselves, they can investigate. Most corp employees are just too damn busy avoiding doing their regular tasks to be bothered to snoop other employee emails. Yes, I agree that it's not like someone is just tasked with reading all email every day. The point is that they can and do when necessary. Once they start reading, they have no idea where the trail ends so they will be reading a lot.
It all comes down to the same thing stated here multiple times, don't send any messages on corp equipment that you wouldn't want to see read aloud in front of your manager/boss or worse a courtroom.
Edit: search "email retention laws" for more precise rules and specifics