Lets say they let me deposit one satoshi.
Over the first week it becomes 2 satoshi.
On the second week it becomes 2^2 = 4 satoshi
On the third week it becomes 2^3 = 8 satoshi
Now , after I leave it for 51 weeks , which is around a year , I decide to claim my payment.
They owe me 2^51 satoshi * 1/100,000,000 satoshis per Bitcoin = 22.5 million BTC.
That means , even if one satoshi is constantly doubled and reinvested each week , by the end of a year they will have to pay more than the entire Bitcoin economy will ever produce.
The only way they are able to repay is by stealing money from the next person. Sooner or later , when deposits slow down and many withdrawals are due , they will just shut down , like every other Ponzi scheme.