I thought it wasn't.
AFAIK you aren't allowed to show subliminal images, unless you tell the audience and show the images. This came in after the Labour party tried to use them in party political broadcasts.
Tomorrow's World did a thing about it a couple of years ago, they showed said broadcast and a horror film which used them and then had to show what the images were.
it does seem to me that big brother is incredibly destructive to the contestants' mental health...although there are probably coping mechanisms they make which aren't shown on tv because they don't involve nudity, alcohol or talking about breasts. but i still think it's an incredibly bad idea.