But you wouldn't be? This is a product literally called "InvestDirect" where private individuals are buying stocks and assuming all the risk.
If you don't want to offer margin on virtual currency products, more power to you, that is your risk. But if customers are buying with cash, it seems pretty suspect for the bank to claim that is increasing the bank's risk exposure.
Honestly it wouldn't be the worst idea to have a "Net Neutrality" for brokers.