The U.S. isn't a signatory to the convention and nations exist in a state of anarchy, one cannot (in an official capacity) impose their will over another without consent. There are other agreements between Turkey and the U.S. which would permit any and all passage, so in the absence of the Montreux Convention (which is indeed absent in this case) the U.S. can sail a carrier right on through without breaking any agreed-upon rules.