What they are drinking is Senburi(Swertia japonica) tea. It is very bitter, but good for your health, and is often used as a penalty game in Japanese TV programs.
I think its some kind of herbal medicine drink, they used something like that for a morning musume game and ther reactions were similer, also while it's a punishment, they are not being harmed due to the good herbal content, but thats just what I think it is ^^
