Why do they do that?

As an experienced traveler, I regularly run into a situation where the housekeeping staff will put the plug in the tub and leave it there.  Why do they do that?

My reasoning is that its pure laziness on their part, mostly because I am the guest and shouldn’t have to be greeted with a tub full of shower water after a couple minutes of letting it get warm.

Anyway, just a small gripe I had to get off my chest today.