Agreed. I’ve never really understood why people leave it running. It doesn’t take much effort to push the toothpaste down the sink afterwards. And your hands will be minty fresh!
This goes on my list of environmentally related bugbears including people who leave their car engine running while stationary (eliciting glares from me when I walk down the street), and people who chuck their rubbish on the train tracks because ‘it doesn’t count if it’s on the tracks’.