Our office has a boiling water dispenser and I do use this to wash out my coffee cup prior to making my next coffee.
Today the person behind me muttered something under her breath when I did this. I thought she was feeling impatient just because I was delaying her cup-of-tea workflow. But she then asked me to think about how wasteful it was to pour boiling water down the drain. Then she used boiling water dispenser to make her tea.
I thought about it. At a universal level I don't see what the difference is.