It has nothing to do with U.S. vs. Foreign carriers. U.S. regulations mandate that U.S. bound international flights abide by the "stay in your ticketed cabin" protocol. I've been admonished of the same when visiting my partner in J from the F cabin on CX. I think they are particularly fussy when FAMs are on board. It's fatuously arbitrary, though I'm sure there's was mindless bureaucrat somewhere at some point who thought it was a great idea -- much like the mesh curtains that flood the F cabins full of galley light on our domestic carriers.