Ripping off investors

Looking into ways of making a few personnal investments, I stumbled upon http://www.currencyshares.com/ from Rydex Investments. Those funds offers to invest in foreign exchange currency. You buy euro, euro goes up, you win money. quite simple. So where is the said ripoff? The catch is that those funds don't pay you interests on the foreign currency you invest into. To see how bad this is, consider the fact that an entire class of trading strategy (fx carry trade) is placed to grab the interests rate *differential* Now just removing the whole interest rate of one currency is bold.