I don't understand why this argument gets thrown about so often. Obviously not so much about neo-nazis in particular, but whenever a comparison is made to LGBT people. And before anybody jumps to conclusions, I am not about to argue that sexual orientation is a choice.
Even in the face of overwhelming evidence of all kinds, from all sorts of sources, there are people that seem to honestly believe the earth is flat. There is no way to make a reasoned decision to believe that. It must be something they are not in control of. It could be something they were born with, something in their experiences, or both, but it's clearly something they are not rationally deciding.
I'm not certain it can be said that the neo-nazis are definitely making a choice. It seems to be a pretty vehement emotional response, which would indicate it's not.
I don't mean to say we should tolerate neo-nazis in the sense that we just let them do their thing. But I do think we might be better off treating them as people that have some predisposition to being neo-nazis than as people that just decided to be one.