I guess fundamental thing with the original post is that exchanges just make money from fee's for providing a service. Banks make money with other peoples money, they have different business models, although banks do have fees as a revenue stream also. Bitstamp probably just holds peoples Bitcoins in cold storage and makes backups. If Bitstamp was to start using other peoples Bitcoins to do things, then that could be illegal potentially (I'm not a legal expert but that's an educated guess). Which in turn would hold them liable.

Then again coinbase might be a different story as they are not just any exchange according to them, as it does function slightly more like a bank account and who knows what they are up to.