Hi MrEarl
There are two good reasons why banks might consider such a ban.
Firstly, customers can claim that a refund if the goods or services were not delivered or if they were defrauded. So I would imagine that after the cryptos collapse, the buyers will seek to recoup their losses from the credit card companies. The best way to avoid such complaints is to not allow them buy cryptos in the first place.
Secondly, if people pay real money for something which turns out to be worthless, they often don't treat the debt seriously. I am told that Credit Union's biggest losses are for wedding loans when the marriage doesn't last. Neither of them wants to pay the debt. So if you buy €5,000 worth of Bitcoin with your Visa card, when Bitcoin crashes, you might just welch on your debt.
Brendan