What you seem to be missing is that the OP tried to buy them then, Coinbase did not process the transaction in a timely manner, and subsequently bought the coins for the OP today after the OP's post at the original price, having already breached the purchase agreement.

This is a pretty close to being a classic contract law case, except with bitcoins rather than tangible goods. If this goes to trial, the outcome is not likely to be favorable to Coinbase.