[aucontraire]

Not true that there is any sort of official rule to this effect. I've slept in a lot of crew hotels that had only semi-decent curtains.

It is true that well-behaved airlines, who take an active interest in fatigue prevention, do try to book hotel rooms with blackout curtains and good sound proofing. This is however not a universal behaviour and not all destinations where layover is necessary have any hotels with these qualities.