When get get cash from an ATM, the bank associates your name with the unique ID on every bill. Since banks are tightly-regulated institutions, you can bet that they pass all their logs directly to "higher ups". When you buy something at a stire, at the end of the day, the first thing any retailer does is deposit all their bills in a bank, which again scans the unique numbers on every bill, and this data is probably again passes right on up to those who have an interest in tracking you. In rare cases, a bill may change hands a couple times before ending up in the hands of a retailer, but modern data mining techniques can trivially follow two or three hops, and I assure you that there is absolutely no way the NSA would simply pass up having full access to such a valuable trail of information. So, in other words, cash is no panacea of anonymity. With cash, not every transaction is logged as explicitly as with BitCoins, but at least someone has to do a little bit of work to attach your real name to your transactions.