This isn't unique to Wales, indeed, the worst drunken behaviour I've seen was in Norway, Iceland and Ireland.
You forgot Denmark! Was in Copenhagen a few year back and about 100 "football" supporter were at the main train station, basically running am-mock, throwing and smashing bottles. I expected the riot police to charge, but the police just stood looking very relaxed and waited for them to stop, then the cleaning staff arrived, as this was regularly Saturday afternoon event. (which I think it was, judging by rest of the commuters complete non reactions)
On Cardiff. There was a more serious isolated violence in the city years back, but now its much less violent but far more widespread low level stuff on Friday/Saturday nights. But as a society in the UK any confrontation these days is out of the norm. Personely I find the city centre on Fri/Sat very off putting, Hen/Stag/Beggar central.