I'm familiar with the news story, and I'm not debating it's very likely that some people were drugged. But free-flowing alcohol creates an environment -- a "haze," if you will -- where such things can occur. People have drugs like GHB slipped into their drinks in the States, too ... sometimes by another patron.
BTW, the paper that broke the story, the Milwaukee Journal Sentinel, is in a state where the population is consistently in the top three for binge drinking. So any contention that a Wisconsinite "just had one drink" causes my eyebrow to twitch. I say that as a Wisconsin resident.