The Customs User Fee is a fee assessed by the US government. Passengers on flights from Canada previously were exempted from payment of the fee. Congress repealed the exemption and imposition of the fee started on November 5, 2011. AA didn't implement a "custom user fee" but began assessing the "customs user fee" as required by federal law.

I assume your ticket is a return ticket and the fee is being assessed for your entry into the US on your return.