I just started to notice this the other day as well. I figured since they were a BTC/USD exchange they would just establish a market rate themselves. The only thing I don't like about coinbase is you can't put in reserve buy/sell orders at a given market price. Meh, other than that I like coinbase a lot, very stable and reliable.
I've developed my own tool that does buy/sell/stop loss orders through Coinbase, it works completely for me so far, but is only prepared for small scale deployment. Anyone want to help me beta test it?