Well, imagine there was a shop that is selling illegal drugs only. Would it be ok for your bank to transfer money there? Nope, they cannot, illegal activity is obvious. In this case I don't think it would be forbidden to buy crypto. It's just that the bank is fearful as the regulation is not in place yet and they hate risks. Especially when it comes to higher amounts of money there is always the questions of avoiding tax or money laundry and that is definitely not allowed. So the bank simply does not want to be affiliated with that. It's their right to do so. One does not need to like that, but I would understand it. And that is why you have to find another way to send money to other places first and then from there you send it to exchanges.