Definitely, and so are most free email services. Try blocking Gmail, on the other hand, and your users will be somewhat upset. Sendgrid doesn't have that kind of weight behind them.

Of course, but it very much a "do as I say, not as I do". i.e. google applies a zero tolerance policy to receiving spam from any domain, but is likely way above any absolute threshold they impose on incoming messages for the spam they emit.

It is what it is.

I'm in favour of anti-trust action against Google and Microsoft for their outright anti-competitive anti-spam practices, but that's going to be a long and hard up-hill battle for anyone attempting it.

Meanwhile, the rest of us will have to accept things for what they are and deal with them as well as we can.