Also from their Terms of Service:

> Coinbase may cancel or reverse potentially high-risk buys or sells of bitcoin, including those made using reversible payment methods.

Could very easily be that your transaction was flagged and then reversed. Your BTC was returned in full. If the price had gone in the other direction I doubt you would be offering to compensate Coinbase for the difference.

That's fine that they can reverse transactions that are fraud. However after investigating the transaction and determining that it isn't fraud, then they should be on the hook for honoring the transaction. Incorrectly flagging legitimate transactions as fraudulent is their problem, not the customer's. They should either improve the algorithm or provide additional controls that give them confidence that a transaction is not fraudulent (e.g. two factor authentication. linking bank accounts and confirming a person's identity). If a person has completed all the security requirements Coinbase has, then that person should be able to make a transaction with the confidence actually will clear.

It's a Coinbase transaction and not a Bitcoin transaction that's at issue.