It is gross!
I have always assumed its a way of using up the wee that has to be released.. peeps that are down the very front at these things are there for the duration..
its either stop your fluid intake.. or pee where you stand!
Idiots that are jealous of the people nearer the front then think its cool to throw it!
TBH its other missiles that worry me more.. Daughter & I had to beat a hasty retreat away from the front when we were at The Prodigy gig last year