the bedroom door

I don't understand what the big deal is. When I was a child we were never allowed to close a bedroom door except when changing clothes. Bathroom doors were not locked - mostly because there were more butts than toilets. The open door allows parents to seem like they are supervising even when they are busy somewhere else in the house. Now that my son is not in danger of being smothered by kitty, I open his bedroom door after he is asleep. I can check on him, the kitty can sleep on him, the air conditioning or heat can circulate better and I have a better chance of hearing him if he gets up in the night. I also like to leave my bedroom door open (after grown-up play time) for all the same reasons. I have my bath robe by the bed for quick cover-ups. My son knows to knock at the door and not enter. So why does M keep raising an eyebrow and telling me to close his door? Why do I wake in the morning to find a trapped kitty and my bedroom door not only shut, but locked?