im not surprised the police are all over them. drinking on the streets is illegal in barcelona. i got a small fine for it myself in my first year living there.
theres an unwritten rule that you should at least attempt to hide the fact you have a beer in your hand when the police go by. not swigging it down in front of them and flinging cans on the ground. id fully expect any group of people openly swigging beers in placa reial to be moved along or approached at some point. add in the noise of them lot and the people pushing then of course they are going to find trouble.