I think this map is a bit stupid for this reason: US states are not direct members of UPU, the whole US is one member, so displaying US state boundaries on the lower map doesn't make sense.
There are no separate ICU locales for US states as well, all US is simply en_US. And the date format is closely tied to language, not necessarily the country, I guess minoroties use the date formats according to their language, not the official state version.
No I think it’s fine and my comment wasn’t sarcastic. US states really are similar to European countries, they are highly autonomous and have different laws and cultures. It could absolutely be the case that one state uses m/d/y and another uses d/m/y. We have states where you drive on the wrong side of the road (well, territories) and states where you aren’t allowed to turn right on a red light, and states where people call all soda pop “coke”, etc.
I think it really boils down to cultural homogenization and borders. Prior to the eurozone, Europeans couldn’t just walk across the border and live in another country. But the US has had that for its entire history. So perhaps cultural practices in the states homogenized earlier than in Europe; also language.