It's been a few years since I've flown a US carrier trans-Atlantic so I don't know if they still do this, but AA definitely used to make announcements that passengers should stay within their seated cabins for security purposes. The policy started just after 9/11 around the same time Air Marshalls were introduced. Now that I think about it, I think European carriers (I'm thinking BA in particular) made announcements specifying the thing on Europe-US routes, but specifically citing the US authorities for the requirement. Interesting that Transport Canada didn't institute similar measures.