The issue is that Mr Sanders and his supporters believe or act as if they believe that Wall Street are, almost by definition, wholly illegitimate players on the American economic scene and that HRC, by virtue of speaking to them, betrays democracy and decency themselves.
I realize that Mr Sanders says the issue is the speaking fees, but that's a red-herring; people who speak to business group get fees; it's her very associating with Wall Street that makes her unacceptable. The problem with this thinking is that it's not much different from the claim that presidents who choose dialogue with our more ingrained adversaries are illegitimate presidents.
The amounts of the fees, too, are an irrelevant dodge, and a comparison of them with the Sanders family income...a recent meme bandied about as if it's a legitimate comparison...is not meaningful. It's as if I were to argue that, say, London School of Economics visiting professors' speech fees and those receiving them are tainted because I, myself, have been paid to speak only by Chinese universities whose budgets are well smaller.
Unless one is certain that the financial industry must have no role -- not even a far more regulated role in a proper America, that speaking to Wall Street groups therefore is in itself disqualifying, the fees canard is unworthy of those making the claim.