That's a whole lotta' pee, I don't care how many concert goers went in a single jug. That bottle is clearly filled to the top, intentionally.
So, why would protesters do such a thing? Simple, it's not urine. If the Seattle facial routine is running it's course, it's actually a compound of lemon juice, vinegar, and few other things I've forgotten about. It's a homemade pepper spray treatment that just might give you back your eyesite long enough to get out of harms way.
It was used on a few surprised reporters in Seattle, who I think were a tad confused as to it's purpose, and reported that protesters were throwing urine. Or, maybe that really was a total falsification made up and propagated by the police. But chances are good that it's not urine.