My question to you is not trying to be offencive to anyone:
would you like to sleep in your room if you had to wee and poo in the corner?
My snakes use to do it too, my female got it in my mum's, hand bag once. hehehe
I think that when they get out of their enclosure and move around a bit, the movement stimulates the faeces(poo) down to the cloaca(bum) and then they need to go and because they don't realise where they are they just go.
I now have that female toilet trained, whenever i take her outside she goes to the loo, she very rarely poo's in her cage, she is happy to sit in there and only tries to get out when she is hungry or needs the loo. But seriously can you imagine sleeping in the bathroom next to the toilet, no a very nice smell. You also might not notice they have pooed either and you might not remaove it till you get home.
I think it would be like being locked in the bathroom and the waters been cut off so you cant flush until someone gets home.
Hope that helps
Cleopatra