Those system don't have to follow the same regulations as banks and thus have many more ways to extra money that might not be as obvious. Money when converted to tokens and credit are not money in the eye of the law, even if it is used identically to money and is the only currency allowed.

If you are lucky they will return any unused credit automatic to your credit card without any fees. I would like to know how common that is and what kind of terms and conditions most such system has. My cynicism tells me its about as abusive as they can get away with but to be fair I have not tested it (since I either refuse to prepay on principle or only get the exact amount I have decided to already spend).