You're thinking too small: if they nickel and dime a million people that becomes 75mm EUR. How many people are going to cause a big enough stink over 75 EUR though individually? Not really anyone, so they could get away with it - maybe if enough people posted similar stories then it would gain interest from enforcement agencies.
That's still a million customers who went through the friction of setting up a new bank account, and whose first impressions are now net negative.
They sometimes refer to their referral bonus system as a game or challenge, that says it all really.