The constant flowing water and exposure to the elements provides a lot of mechanical action to wash away bacteria. I've also never actually seen anything gross stuck onto the water spout, so I'm not too worried about it.
There's chloramines in the water to deal with things like that anyway. Nobody is scrubbing out the water mains beneath the streets - they're not 'clean' by any common definition of the word, but they are perfectly acceptable - they're sanitary due to the disinfection compounds in the water.
