The risk of charge backs for banks is not really a good argument, charge backs for SEPA (eurozone) transfers initiated by the account holder himself are not a thing that exist as far as I know. You can't reverse a small erroneous transfer if you initiated it as the account holder (in contrast with credit card payments in the US for example).

Furthermore, I'm not exactly the first person to buy crypto through the largest crypto exchange in the world (Coinbase) with such a Portuguese bank account. They know this Coinbase bank account, they know what it's for (buying crypto) and they know it's not a scam.

They could easily decline a single card payment instead of suspending the whole account.