It appears you had a connecting flight. Passengers virtually worldwide are not allowed liquids exceeding 3 ounces. The customs area is not secure since you have access to your checked luggage which can legally contain items such as knives, guns, liquids ect..
Leaving customs for the above reasons you are subject to the same rules as if you were entering the airport. You could have placed the bottle in your checked bag and everything would have been fine. If you did not have a connecting flight and exited to the non sterile area you would also have been fine.