In the movie the Piss drinker is a high Japanese official who has had some trouble with a guy who's been talking shit about him and the HQ. Special vitriol has been shown against the water cleaning system. Piss Drinker shows him he ain't having none of his shit by downing a lovely glass of kidney juice straight from the WC.
Why on earth he did not make the other guy drink the piss is beyond me as by not doing that he missed out on a more ironic justice and more importantly, not having piss in his mouth.