It is not a ponzi scheme. The exchange does not pay this interest / yearly fee out of other's deposits. The exchange will not collapse if new customers do not come in.
It might not be the standard definition of a ponzi scheme, which is why I called it a legal ponzi scheme, but the core structure is almost the same.
Third party wants to put
your coins to work; check.
Third party
promises you a reward per whatever time frame; check.
Third party
can mess up with coin loss for
you as result; check.
We are talking about a party that doesn't have much past history in this field, which is what I consider to be an extra risk. The problem with this is that it works till it doesn't work anymore, and when it doesn't work, you have no insurance, no backup fund, nothing. Perhaps I'm too paranoid, but we shouldn't take risks with semi high probability of losses.