The explaination for potable toilet water is that you are taking water from the cistern, not the bowl. But have you ever looked in a toilet cistern? Nasty. Also that doesn't explain how you get potable water from toilets like the one in Quonset Garage. How fresh snow in a subfreezing environment is unsafe, I don't know.

Melting/boiling snow should be combined into one action, and toilet water should be unsafe.