rather than go to a bathroom like a civilized person, Bieber -- wearing pants that should literally be illegal -- whipped out his junk and whizzed into a yellow mop bucket used to clean the restaurant's floors ... meaning whoever's job it was to mop the place up had to physically change Bieber's disgusting piss water.

And the worst part ... Bieber's friends act like the restaurant should be HONORED that the singer decided to piss there. It's revolting.

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