I suspect it's simply has to do with the fact that his views very much go against convention (other LCHF proponents around the world face much the same reaction). And since the evidence for it it being, at the very least, not dangerous at all and potentially quite beneficial, a lot of people are left with little more than irrational ad hominem attacks on Noakes and people who happen to eat this way.
That all being said, there are no doubt banting and Noakes supporters who are no doubt verging on the religious about the topic, which is just as silly. I honestly don't know why people get so damned worked up about what they eat. It's insane.
So people think it's a dangerous diet because it is unconventional? Maybe the bad rep originates from media created by those who are losing financially over banting popularity, and people are regurgitating the information from those articles.

