Well, this might be a tad ramble-y, but I think there's a reason for that initial mindset (I'm not going to touch on the knock-on effects you hit on already): the colonization model for the US (and to a somewhat lesser extent, Canada) was quite different from that of Spain and Portugal.
Y'see, the reason those countries have such a mixed population is because the initial settlers there were almost exclusively men, with the same....appetites as men are wont to have. And without any women coming over from Europe any time soon, they had to get accustomed to holding relations (however willing or not) with Native or African-descended women, thus leading to accommodating a "pigmentocracy" which recognized mixed-race persons.
The English, however, did ship entire family units, or even communities, en mass to their colonies from the beginning for the most part. This meant that the entire demographic-cultural bedrock of those countries was set to be distinct from the get-go, which wasn't helped by a much different (i.e. violent
and prolonged) broader interaction between those settlers and the Native Americans. Even so, both the US and Canada do have mixed-race communities going back centuries (respectively,
Melungeons and the aforementioned
Métis), it's just that the different colonization pattern I mentioned meant they were a relatively small group overall, not the norm.
FWIW I don't think the whole Latin-Germanic identity has anything to do with it inherently, since South Africa has a significant mixed population originating from Dutch settlers. Furthermore, the argument that the Iberians were more prone to interacting with persons of different skin color is more than a little spurious to me, since those persons were the OTHER side during the Reconquista and/or Inquisition...I fail to see how/why that would promote racial tolerance at all. I see it it boiling down to a mixture of the aforementioned circumstances with possible religious mindsets on "ethnicity" (such as it was back then) that crystallized over time.