If you are claiming they are backed by USD, and you are creating them out of thin air, then that's a scam.
Other coins are only valued based on how the market values them. There is no fraud there; you are not claiming that you will redeem them for something else, they just represent themselves and are valued based on how useful people find that particular kind of cryptocurrency, plus hype.
MtGox was also a scam by the end, since they were trading on their exchange bitcoins and USD that they didn't actually have for people to cash out.