Because it's leveraging huge existing monopolistic power. I'm not a lawyer, but it's the abuse of power that's the problem.
What's the problem with that? Simple, let's say you dislike <X>. Now Google wants <X>, but knows some or even most consumers don't like it. All Google needs to do is implement <X>. Disliking <X>, you switch to Firefox.
Google now has <X> and sites start using it. Suddenly, sites that require <X> don't work in Firefox. Firefox loses market share because it doesn't support <X>, Firefox now implements <X>.
Disliking Firefox, you change to IceFox++, which dies after it's lone maintainer dies in a sky diving accident.
Enjoy your options.
> There are lots of email services that aren't Google
Sure, and if you don't have a Gmail address and send it to a Gmail address, good luck getting past the Gmail filter. Why not join Gmail, for the supreme mail experience? Sure you can use FastMail but your friends all use Gmail.
Why worry whether your mail will get a pass from Google. Just join it. It's the optimal choice. In fact, it will soon be your only choice.