It's the FAA Modernization and Reform Act of 2012.
https://www.gpo.gov/fdsys/pkg/CRPT-112hrpt381/pdf/CRPT-112hrpt381.pdf
Search within it for definitions. If I recall, it's about page 80 or so.
T
Yes it's page 68. I'll save people the hassle.
SEC. 336. SPECIAL RULE FOR MODEL AIRCRAFT.
(a) IN GENERAL.—Notwithstanding any other provision of law relating to the incorporation of unmanned aircraft systems into Fed- eral Aviation Administration plans and policies, including this sub- title, the Administrator of the Federal Aviation Administration may not promulgate any rule or regulation regarding a model aircraft, or an aircraft being developed as a model aircraft, if—
(1) the aircraft is flown strictly for hobby or recreational use; (2) the aircraft is operated in accordance with a commu-
nity-based set of safety guidelines and within the programming of a nationwide community-based organization;
(3) the aircraft is limited to not more than 55 pounds un- less otherwise certified through a design, construction, inspec- tion, flight test, and operational safety program administered by a community-based organization;
(4) the aircraft is operated in a manner that does not inter- fere with and gives way to any manned aircraft; and
(5) when flown within 5 miles of an airport, the operator of the aircraft provides the airport operator and the airport air traffic control tower (when an air traffic facility is located at the airport) with prior notice of the operation (model aircraft operators flying from a permanent location within 5 miles of an airport should establish a mutually-agreed upon operating pro- cedure with the airport operator and the airport air traffic con- trol tower (when an air traffic facility is located at the airport)).
(b) STATUTORY CONSTRUCTION.—Nothing in this section shall be construed to limit the authority of the Administrator to pursue enforcement action against persons operating model aircraft who en- danger the safety of the national airspace system.
(c) MODEL AIRCRAFT DEFINED.—In this section, the term ‘‘model aircraft’’ means an unmanned aircraft that is—
(1) capable of sustained flight in the atmosphere;
Where does Congress give FAA ability to "
legally" force rc model registration as the op stated?