It looks like this from the inside:
Would you be able to use a toilet if it looked like everyone could see you, even though you knew they could not? This is a taboo because even though people could not see us, the mere thought of them seeing us would make us hesitant. In other words, simply thinking about doing this is embarrassing and so we don't want to even think about it. Perhaps, that is why we have so many euphemisms for using the toilet: using the john, the restroom, the bathroom, the lavatory, the men's room, etc...
