In Bitcoin, fees are earned by the miners. Who earns the transaction fees in Ripple?
The transaction fees in Ripple are simply destroyed. There is no mining.
We had to use ripples because we couldn't use dollars, both because dollars in Ripple can only be represented as debt (Who would owe them to the system? Who would they go to?) and because Ripple is intended to be cross-currency (How would you manage exchange rates?).
The beauty of ripples is that the network can destroy them and they require no central authority. In the future, if ripples become unusually scarce or plentiful, the XRP cost of transactions can be adjusted by consensus.