A new thread on Reddit suggests that Coinbase monitors how you spend your bitcoin. Specifically, if it suspects you are involved in transactions involving illegal drugs, Coinbase will close your account.
"Coinbase is a regulated Money Service Business under the FinCEN division of the U.S. Treasury Department and as such, we are required to review accounts in order to ensure compliance with regulations and/or our Terms of Use."
https://www.reddit.com/r/Bitcoin/comments/2q90m9/coinbase_is_monitoring_your_transactions_poorly/
"Coinbase is a regulated Money Service Business under the FinCEN division of the U.S. Treasury Department and as such, we are required to review accounts in order to ensure compliance with regulations and/or our Terms of Use."
https://www.reddit.com/r/Bitcoin/comments/2q90m9/coinbase_is_monitoring_your_transactions_poorly/