Did you take your rabbit in recently? Rabbits feel safer surrounded by their own smell - when a rabbit is in an unfamiliar environment or when they feel stressed or ill, it is not uncommon for them to sleep in their litter box. So, either your rabbit still isn't used to his new home, either (if you've owned him for a long time) he's just easily stressed and feels more relaxed in his litter box.
In Strax case, it's probably just because they understood where the room service was and they decided to become lazy XD.