There's much I don't know about email, so take what I say with a grain of salt. I imagine that maybe this could simply be a low priority issue for Fastmail because such a restriction would not be a protection for their customers but rather a restriction/disservice to them to potentially protect everyone else.

I also wonder if there are superusers that have a legitimate use for sending emails that have a different "From".

Something to think about is that, looking at the postal mail it was designed after, I don't imagine a postal office would reject me if I tried to drop off mail authored by someone else. They don't check the "From" in the envelope with my ID or anything. In fact, many envelopes don't even have a "From", and you don't even have to face a human when dropping off your mail. All the postal office does is provide access to the global delivery network for a fee.

It might be more apt to think of email providers likewise as network providers that allow transparent access to the global MTA network.

Both postal and electronic mail rely on signatures for proper authentication. It's only that electronic mail's (cryptographic) signatures are more secure but more difficult to use by laymen.

Maybe this issue ought to be thought of a similar to how illiterate people sign paper documents by making an "X". I imagine it's trivially easy to spoof documents supposedly signed by them, and even mail them. I wouldn't blame the postal office for accepting such spoofed documents.

Computers being relatively new and all, perhaps it isn't that bad to think that most of the world is still computer illiterate even if they think otherwise because of their ability to use point-and-click interfaces designed to be used even by illiterate young children.

What I think is needed is better computer education.

As to where this expectation for "From" to be validated comes from, I imagine it's something we've grown accustomed to from our use of centralized services. It would be really bad if a message on Facebook or Twitter could be spoofed, but those services are centralized, so restricting their users equates to properly protecting their users. Email, however, is decentralized. That's a good thing, and the proper way to do authentication in an decentralized service without making it more centralized can only be by non-spoofable signatures and not by trusting validations from independent service providers.