Bitcoin is decentralized, but it's definitely not private. Every transaction is recorded publicly in the blockchain. Some folks are very good at backtracking to figure out how funds are flowing through the network. And sometimes it's not even hard, especially if you reuse a wallet's address for receiving funds across multiple transactions.

In particular, there was an article alleging that Coinbase keeps a list of suspicious wallet addresses, and transacting with them will get your account banned. @gwern (who posts to HN sometimes) is reportedly one of the folks whose address is flagged as suspicious: http://truthvoice.com/2015/05/coinbase-accused-of-tracking-b...