(I wonder if British law considers a corporation to be a person to the degree that US law does, or if this plural view of a corporation is pervasive in law as well as in grammar.)
Edit: For example, here's what the Guardian's style guide says on the matter:
http://www.guardian.co.uk/styleguide/c#id-3022716
Of course, with something as flexible and constantly evolving (and as used and abused) as the English language, it is usually possible to find examples and counter-examples for just about anything. There are also edge cases; the Guardian refers to police forces as plural entities, I believe. Suffice to say, when I was running a back bench, singular was the order of the day when it came to company names.
But then I never won any awards for grammar.
But for me, when speaking and writing consider companies to be groups of people, and so I use plural. And I think that is common over here in Britain.
That should be explanation enough.
I'm intrigued that you are ready to rule out the contributions of a class of people who manipulate the written word for a living (and debate usage among themselves to the point of distraction!)
ASDA? Well, I'd use "Asda" since it is pronounced as a word, not four initials. But the company itself still seems to be struggling for consistency on that...
I'm not a linguist but parsing these sentences as an American reader, I feel a bit like this page's examples are playing games with word order and omission. The cited "prestige" grammar sounds less intuitive to me not because of the pronoun used, but because of word order and words that are left out.
Example cited as correct prestige grammar:
> They didn't give anyone that worked less than she a raise.
That sounds a little weird to my American ears, but "worked less than she did" sounds totally correct.
"Worked less than her" (cited as correct non-prestige) sounds a bit casual and informal, not sounding too jarring but not what I'd expect in decent writing. Similar to the other example of "us commuters". If I'm talking to someone I wouldn't blink if they said this, but I wouldn't see it in the New York Times. (Though this also reminds me of phrases like "me too" or "it's me", which despite being inconsistent with distinctions between subject and object in other phrases, you'd hear a lot more than "I as well" or "it is I".)
> Mary and him are late.
Sounds very wrong to me.
Thinking back to my childhood it was pretty common for kids to be a bit "confused" about using pronouns this way before 10 years old or so, so maybe there is something to the author's statement that kids learn the non-prestige form and then the educated ones are "corrected" later.
> Mary and he are late.
This still sounds weird. I'd say "he and Mary are late".
> her and us
> she and we
These sound pretty clumsy regardless of which is supposed to be used.
An English person, writing English in an English way, and you say you're not going to continue reading. Do you require him to write like an American? Why should an English person, writing their own language, have to follow your conventions?
"I didn't know"
Well then don't start shouting your mouth off! If you don't know, keep quiet.