My casual adult practices and lost memories would definitely disappoint my maternal ancestors... My mother was from a mixed family of blue-bloods from the US northeast and German immigrants a couple generations removed. She probably married "down" with my father, my inculcation thus diluted. She didn't talk about it much, but I think there was some pressure to conform to those blue blood ways within her extended family. I believe her sister was involved in the Daughters of the American Revolution and was also put through some sort of finishing school when young. I hear an odd echo of that aunt whenever I hear Martha Stewart speak. I remember events with very formal table settings when I was a small child, though I have forgotten many of the procedural details for upwards of 5 utensils per place.
All of this is to say: it is a formal cultural practice at least in parts of the US, not some absence of training. I would also believe it has some origins in a display of disarming, much like a hand-shake and other rituals. I wonder if anyone from the UK can comment on whether there are notable regional or class-based practices and any that seem to match the American method described here. I would expect there were British roots, though it might have died out.
More practically, the US method limits eating to a polite pace. Out of context, European and East Asian eating methods look extremely crude to an American eye, with people shoveling, lapping, and slurping food towards their faces. In context, I of course realize they are culturally expected and my American way would seem odd or perhaps even disdainful.
As an aside, I found it interesting to learn how Thai people use a fork and spoon. They use a spoon in their right hand much like an American uses a fork, except they may keep a fork ready in the left to assist in loading much like a European uses their knife (but with hands reversed). But, if an item needs to be cut, the spoon is suddenly a knife without any hand switching and the fork may control the item as it does in American hands.
It would indeed be interesting to know if there used to be regional variants in the UK. Actually, do you have a feeling for whether this has strong regional ties in the US, e.g. to Virginia vs Boston?
Obligatory book plug: https://en.wikipedia.org/wiki/Albion%27s_Seed (as long as you have a decent tolerance academic writing) and http://www.colinwoodard.com/americannations.html (some overlap, much less formal).
I eat my peas with honey
I've done it all my life
It does taste kind of funny,
but it keeps them on the knife.(In case you circle back to this vital topic!)
Edit: spent 10 minutes googling for the right Miss Manners link, but failed. However I may be wrong on the shovel thing, that may actually be encouraged, I'm not sure.