But that doesn't explain the many FAs on US carriers who deny people (even some from Canada) who do not have a very specific sticker on their carseat, even if it is an approved model.

Perhaps it is a training issue amongst FAs, but I have seen (and read about, and experienced) it happening on US carriers. I've even seen it happen to American travellers.

When you say 'foreign', I assume that you mean 'non-US'. Some of the same models are sold in many countries.