Many guests indeed take the view that consumables are permitted to be consumed, and take them home.

The writing pens and notepads commonly provided in rooms were a common example. Some people (not me) even brought home the extra box of tissue paper.

So, yes, draining soap or shampoo is seriously a practice done by some guests, who were formerly in the habit of bringing home the individual plastic toiletry portions.

As a life member of the Sierra Club (see my profile), I do see the pros and cons of the switch to bulk toiletries. Ultimately, the consumer will have the final word.