Can't see what the fuss is about
If someone emails me they do so with a reasonable expectation that I will use an email client of my choice to process and read that email. It might be that I choose to view the email in plain text (SMTP headers, MIME headers and raw HTML, etc). Or I might choose a client that only shows me the formatted HTML part, hides the headers, handles attachments etc. I might have everything - spam and viruses included - dumped into one big "Inbox", or I might have my email system filter them in some way. And I might have my email system, should I choose to do so, provide advertising alongside it based on the content of the emails.
By sending me an email, the sender has chosen to accept this in using my email address in the first place. All of the above are in my control, not the sender's. It is not within the rights of the sender to determine whether my client is Outlook, Thunderbird, Gmail, Hotmail or just plain telnet to a POP3 server (any more than it's within mine to dictate their choice of sending client).
That I might choose to use an ad-funded service, and that I might prefer to have a handful of vaguely relevant ads rather than a pageful of random "hopefully one of these will be of interest" ads, is my choice. Those ads aren't shown to the sender, the sender isn't getting spammed or having their details sold to advertisers.
When I send an email to an Outlook user I am indirectly supporting the business model and practices of Microsoft. I might not like this, but it is not within my rights to sue Microsoft because I object to this happening even though I never agreed to a Microsoft EULA.
And, of-course, anyone who is *really* bothered about this would just encrypt their email so my choice of email system can't pre-/post-process it in any way. Or they could freely check my email account's MX records and determine where the email is going. If they're too lazy to do either, that's their problem not mine.
Sorry, but Google is no angel but anyone who believes that this is anything more than an exercise for lawyers and for people who think they might be able to make a few quid is deluding themselves, and that kind of people should not be encouraged.