My main point is that people shouldn't blindly invest into whatever asset with money they don't have. In this case I don't see it as a problem that banks are preventing this from happening, because it's their money.
A credit card can be used to get into an adult entertainment show, but they cannot be used to buy bitcoins? I find it strange that banks are okay to fund expenses, but are not okay to fund risky expenses.
The due diligence on the customer should have been done before the credit card was issued. They really shouldn't be trying to micromanage what the customer does with his sanctioned limit.