Because they don't want to get sued.
I thought brokers were in business of trading shares. Why is Scottrade not allowing their customers to buy RCCH shares? If Gene is selling shares, as you claim, a broker would be happy at the opportunity to make money selling these shares.
If they suspect that the company is behaving in a manner not conducive to ethical expectations, you can bet that they are going to stay as far away from it as possible.
When the inevitable end game reveals itself and all of the investors suddenly become irate at how they were duped, lawsuits will fly in every possible direction. Directed at any and all potential sources of money.
Brokers want to be able to show that they were not only uninvolved but made a clear effort to dissuade their customers from making foolish mistakes.
Investors will have a hard time trying to convince a jury that the broker that tried to tell them about the 'manipulation' and tried to talk them out of buying the stock, who even put obstacles in their way when they insisted on buying the stock anyway, was in any way complicit in the scandal.
Why else?