A non resident does not need to register their ATV as you said if you are there for less than 15 days.
Who needs to register?
With certain exceptions, every owner of an OHV shall register their OHV. Registration of an off-highway vehicle is not required if the off-highway vehicle:
Is owned and operated by:
A federal agency;
An agency of this State; or
A county, incorporated city or unincorporated town in this State;
Is part of the inventory of a dealer of off-highway vehicles and is affixed with a special dealer license plate;
Is registered or certified in another state and is located in this State for not more than 15 days;
Is used solely for husbandry on private land or on public land that is leased to or used under a permit issued to the owner or operator of the off-highway vehicle;
Is used for work conducted by or at the direction of a public or private utility; or
Was manufactured before January 1, 1976.
Is operated solely in an organized race, festival or other event that is conducted:
Under the auspices of a sanctioning body; or
By permit issued by a governmental entity having jurisdiction;
Except as otherwise provided in paragraph (d), is operated or stored on private land or on public land that is leased to the owner or operator of the off-highway vehicle, including when operated in an organized race, festival or other event;
Is used in a search and rescue operation conducted by a governmental entity having jurisdiction; or
Has a displacement of not more than 70 cubic centimeters.
"Sanctioning body" means an organization that establishes a schedule of racing events, grants rights to conduct those events and establishes and administers rules and regulations governing the persons who conduct or participate in those events.