Yeah, I think this is done to provide the illusion of choice. The rate limits are high enough to allow personal emails through, but for any mass emails or corporate emails this forces you to use Google. Unfortunately a standard corporate strategy, it’s why corporate office suites are so generic and tend to be from one of the big companies.
Because the goal was to ban third party apps and they don’t want people trying to dodge it. u/spez seems to be personally offended by their existence and wants them gone.