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.
How much can you spend on an adult entertainment show? From what I know the prices range from $40 to $300 on average, and max you can spend if you want a VIP suite access is around $1,000. These amounts are much less than what people are willing to spend on purchasing Bitcoins or other crypto currencies when they "know for sure" that they are making a profitable investment. It's a known fact that the more you invest in case of a profitable investment the bigger profit you will have and thus it is very likely that people will ask for the max credit they can get. ... I have to say that I agree with the banks policy in that regard.