It's the fault of your card issuer rather than the Polish bank. Some Polish ATM's are to be avoided for certain transactions - Euronet is one of them , but most are much of a muchness. The card issuer sets their own rates.
Thank you, it makes sense! It was actually relieving to find that out, because while I wouldn't apply for a credit card in Poland, I can, indeed change my credit card issuer back in my home country. I found one who offers better plans for international withdrawals, so it makes total sense to change.
I however do not recommend to anyone to use Euronet ATM's or Idea Bank ATM's, should anyone query this thread.