By FAA regulations, US carriers cannot have light-blocking curtains, i.e., they must be see-through curtains. On United, which exclusively flies 3-class aircrafts on international flights, it's even worse since there is only a see-through curtain between Y and J, but not between J and F. There is nothing the airlines can do. After all, thick curtains prevented passengers and crew of the doomed 9-11 flights from seeing what was really going on.