It's unclear how, or even if, Niemann cheated. The evidence is, at best, circumstantial: a win by a slightly weaker player, playing black, in a line that should have been impossible to prepare for.

So some see him as throwing a tantrum after an unexpected loss, and blaming it (implicitly) on cheating. We can't know if he's right, and we may never actually know. In the absence of confirmation, some people will think less of him for that.