Ignorant people abound
This is the outcome of people not understanding how science works. Pick a topic, and you'll hear people say things like, "Well, I don't believe in the theory of . I have my own theory of why things happen the way they do." And the thing is, the people who say these things often have no idea what the term "theory" in science actually means, so they think that, just because they hold the contrary opinion, they can call it a theory and give it some sort of weight.
I remember someone (I don't recall who) once saying, "Just because you hold an opinion doesn't mean it's right." And just because a group of people, no matter how many of them there are, dispute something doesn't mean there's a valid controversy about its existence. There was a time when most people thought the world was flat, and they held this opinion despite clear observations to the contrary. And they were dead wrong.
I'm not saying that there is absolutely no chance that wi-fi can't have some negative health effects, but I haven't seen any studies lending credibility to that idea. And until the people pushing it get serious and commission some studies that produce contradictory results (I say "commission" because I have no faith that they'd be capable of conducting the study themselves), I'll continue to view them as nothing more than a bunch of crackpots.