They don't have to follow US regulations not allowing opaque curtains post-9/11. For a few years, curtains weren't allowed at all on USA-based airlines. Now I believe it's a maximum of one transparent, open-weave-type curtain per aisle. Thus on a 3-class aircraft, you either could have a curtain between Y and C cabins, or between C and F cabins, not both.

The US airlines got out of the habit of having them for several years, and I think many flight crew still don't want to use them for security reasons. Nothing that could obstruct the crew's view from cabin to cabin.