If you send a message to an electronic service, the service has to process and often store the message. You might value the processing enough to pay for it, e.g. to scan for spam, allow full-text indexing, back up your local information.
If you don’t like what that service does with your text (scan it for potential advertising options, suck it into a corporate silo that you can’t access elsewhere, make it available to the NSA) you can use a different service (assuming one exists).
However, you might not know that you are sending the message to the service, e.g. my @raggett.net email account is actually Gmail, but you wouldn’t know this by looking at my email address. This case is harder, but if it does matter to you, you should only use communication channels you trust and persuade your audience to do likewise.
It is law in the US that if you involve a third party in communication, you lose any assumption of privacy. Google quoted this law in a filing, partly causing the outrage. This is not necessarily good law in the internet age, but it is the law. Get the law changed, but don’t necessarily blame Google for it (unless they are being hypocritical as they are with web servers on fiber connections).
For well-reasoned rebuttals of the kerfuffle, read a Medium article on the expectation of privacy and Yes, Gmail users have an expectation of privacy (extraordinary that so many of the comments are about a pixelated image).
