I'm not sure anything Amazon said to the FAA could be considered "getting tough." Personally, I don't think the FAA even cares if Amazon moves drone operations overseas.
"In the absence of timely approval by the FAA to conduct outdoor testing," Amazon said in its letter, "we have begun utilizing outdoor testing facilities outside the United States. These non-U.S. facilities enable us to quickly build and modify our Prime Air vehicles as we construct new designs and make improvements."
