I was actually thinking something along the lines of this as well. What sane person in Hillary's campaign would invite her along, with her baggage, without her having some dirt on them?
I can't think of another reason they would take this risk. Even if they wanted her on the campaign, they obviously knew that it would look bad to do that. There's no logic behind it, unless it's for the best for their campaign, and we don't know what the worst was because of it.
But, conspiracy theory, etc. They could just be making terrible decisions.
Or, they believe they will win no matter what they do.
And though the polling doesn't reflect that right now, the demographics certainly do.