Why you think that?!
Offering 4.5% a week of interest isn't a ponzi scheme...
If you let 100 BTC in my hands, I turn it into 105 in one week with purely simple (and hard) work, called "trading"... Am I running a ponzi?! =P
Cheers!
Thiago
Try to scale that trading up to any reasonable level. The margins get smaller and smaller.