Unless people everywhere in the US are flying and encountering this same behavior from an FA, I am not sold on the condemnation of this being a US airlines FA way of life or anything like presented.

Simply put, it was poor form from this FA, and that's all. I would be eager to hear the explanation from the OP some more though. Was this just temporarily while perhaps being caught off guard from unsuspected start of boarding? Did they do this the entire boarding time?