Yea, that's part of the problem, I think most airlines do, to avoid a first flight in vain. There should be a visa re-check at the international hub for those passengers connecting there...

They don't need an extensive 3-month retraining seminar. Just a few email reminders or even something in the check-in system that says all US passports heading to Japan need this visa sticker on it.

I can't remember exactly when the "tourist visa with guided tour" program started, but it was definitely not the whole pandemic by any stretch. The visa criteria were quite different early on, and even if the document style didn't change an agent could easily think that the new tourist program was different. Also, many countries suspended VWPs and effectively closed borders in 2020. UA is a global airline flying directly to many countries and it's unlikely a frontline agent is going to be trained on every change for every country throughout the pandemic. Corporate was bleeding red ink and probably had other thoughts then.

I can't be sure for a destination that requires a visa since I almost always travel to countries with VOA/VWP, but certainly even for ESTA/ETA countries, I get the connecting UA BP at my origin and make the connection with no further verification other than a physical check I am carrying my passport when I board the international flight.

Possibly not. Throughout the pandemic, Japan frequently suspended the validity of certain visas with very little notice. The agent would not only need to recognise a visa, but also recognise whether said visa was still valid according to whatever mood the GoJ was in at the time. Furthermore, this eVisa innovation was a relatively recent one.

I am pretty sure if the OP was checking in at ORD the GAs there would have caught it, they are usually pretty good at the international hubs in checking visas...

You know how they say all accidents are a combination of factors, this one was too, a rather rare combination of a relatively recent visa program, OP's trip originating in smaller airport, and all three people involved missing it...

Except in OPs case there was no visa, eVisa or any document resembling a visa and at no point has an ERFS been considered a valid entry document.

Your profile says that you're "Splitting time between UK and Japan" so I assume that you're familiar with Windrush, Home Office hostile environment policies, and the Rwanda asylum plan? Those are deadly sharp edges.
Turning to more mundane issues such as applying for residence visas and dealing with border agents - The experience is consistently, cheaper, faster, simpler, and more competently and courteously handled in Japan than in the UK. It's not even close. If you have never had to deal with the UK Visas and Immigration office at Lunar House in Croydon, consider yourself lucky.
For sure, there are scandalous failings in Japan - child custody issues, treatment of asylum seekers, housing discrimination. Something should probably be done about the snooty restaurants too, I guess. But to point to Japan as particularly xenophobic, on the basis of time spent between UK and Japan is untenable.

Having volunteered to support people to overcome issues with the hostile environment, I'm very familiar. We have our own issues with xenophobia in the UK, without a doubt. Both the UK and Japan have immigration-related scandals. For your average visa applicant pre-covid, Japan had a much friendlier and much cheaper system. However, it was still less liberal in many ways than UK system. For example, in Japan, spouses of foreigners are not allowed to work. It's a cruel and misguided policy that prevents the spouse from integrating into society.

For me, the difference is that in the UK, there is a legal framework that attempts to protect minorities from discrimination. That's what I meant about the legal edge. Also, Windrush was a major scandal in British society. And because of that, it provoked change. I'm not sure the same reaction would have come about in Japan.

Regardless, it's getting a bit off topic I suspect. Japan's 2020-2022 policy is an emotive issue

United can’t even train their gate agents to board the flights properly and consistently according to policy. Asking them to learn how to recognize visas when they can’t tell the difference between group 1 and 2 is a pretty tall order.

I flew DCA-DFW-NRT in June on AA. They did a perfunctory check at the DCA check-in counter, but the gate agents at DFW re-checked all of the Japan entry documents in detail before we got boarding passes for the international leg. Presumably because those agents were the only ones who knew what they were doing.

Same. We discovered when we went to enter the Star Alliance lounge that our boarding passes printed in Canada didn't show us ongoing to Japan. They called someone from ANA to come speak with us, and they reviewed all our documentation - including the eVisa screen we'd later show at Haneda on arrival - and they then printed us off ongoing boarding passes into Japan.

Just my $0.02, but if you travel to a country that requires a visa and you don't have a visa, then bad on you.

Correction made....

Very much so.

When I consider how much time I have spent researching, checking, and rechecking just before departure that I am good to go for my travels during the last 2 years (the first 6 months Japan made sure that was not an option), I understand the frustration this experience has been, but I have a very difficult time feeling a great amount of sympathy for the situation which was entirely avoidable with a bit of research in to how the visa situation was supposed to flow.

Even going back to my home country has had at least 4 different entry requirements over 2 years, and I think Japan has been at least slightly different every single trip so far. It has been a weird period to travel in for sure, and it is not yet fully back to normal.