Most credit cards put a lot of restrictions on what exactly the bank will allow you to use their money on. E.g. many credit cards place a restriction on gambling. Because, in the end, if you cannot pay back the interest, in the absence of any other solution, the bank ( = the bank’s customers = the public) must foot that bill. And banks, certainly European banks, have a lot of responsibility when it comes to lending responsibly.
I believe that exactly; cashback should be banned; because prices do rise and that at the expense of the poor, students, and other people of few means (who cannot get approved for a credit card). And EU has effectively banned it by placing a threshold on the wholesale interchange fee, which I am honestly very happy about.
Also, topping up is in reality a kind of quasi-spending or manufactured spending, which for natural reasons many credit cards disallow as well.
Debit cards on the other hand have lower fees, and it is your own money. So basically manufactured spending is not an issue. However, I see potential issues with money laundering.
So yes, I believe that from a regulatory perspective, it is both practical and natural that there be a fee on credit card top-ups, for the above reasons.