They all default to ISO sizes for me.

If I format the page size, Libreoffice does offer "Letter" and "Legal". GIMP shows them as "US Letter" and "US Legal" but again they're not the default.

It wouldn't surprise me if most non-US users hadn't seen them at all, and certainly not that they don't realise the US uses a different size.

It's still problematic the other way around - you try to print that PDF on A4, but it's formatted for US Letter.

In most cases it still doesn't matter, either because software defaults to scaling to fit, or else because the margins are large enough that it works out even if printed in true size. But sometimes stars align and then you learn about those weird paper sizes.