It's more like if the person you are sending to hires an assistant to sort and organize their mail for them (which people actually do.) You then go and sue the assistant.
Many people sign NDA's. If you have an assistant read mails covered by the NDA, then the party who sent the mail can sue.
and so on and so on.
I'm not sure how common it is, and it's probably less so now with email, but I'd guess a lot of important people do have their assistants screen their confidential mail. For example, I know politicians often have people help with their mail.
A new cleaning company called CleanU create an app where people can easy order office cleaning by a single press of a button. In the ToS/service/liability contract on page 43, it says in legalize: "any document found by our assistants might be used to improve service and decrease costs".
This might sound as a great way to start a legit industry espionage service, but alas, it would be illegal under several laws. First, a judge would ask if there had been a merging of minds, and thus declare the contract void since no buyer could possible have agreed to such terms. Second, as a product (sale), one could ask if "theft" could reasonable be expected when one purchase cleaning. Since it can't, the contract can be made void in that way. Thirdly, if the intent is to hide an otherwise illegal activity under the assumption that people do not read EULA, that would qualify criminal charges under Mens rea, thus fraud.
In all, reading peoples mail because you managed to get people to click a box during registration is a deal on a very unstable legal ground. Law suits like this will explore exactly how unstable it is.
Cleaning services are also reasonably expected to clean desks. They might even temporary hold confidential documents in their hands while doing so. However, once they start reading the document and interpret them, a line has been crossed which no legal fine print can fix.
It's not that there's an assistant, it's that there's just one assistant reading everyone's mail, they can take all the tit bits to make a story and they've got a perfect memory.
This is one of the many differences between the new and old worlds, the sheer scope of the damage they could reap.
This is all about moving from offline to online.
In the ancient days pre-email it was a non-trivial task to read every single letter being sent in a country, categorize them all and then profile people based on them.
And the countries that did that weren't somewhere you wanted to live.
In the modern age it's a "feature" that companies provide because "if you're not paying for it, you're the product".
Not that I think it's necessarily bad, but boundaries have to be set for this new age and this law suit is simply part of that process. Just look at it from a different perspective.
(And note that spam filters & image preloading are problems that only need to be solved because of the same new trivial cost of automated malicious actions)