And that would depend on what state you live in. According to peopleforbikes, 22 states now classify low powered electric motorized bikes as ebikes (electric bicycle) and no longer motorized.
Additional notes:
THREE CLASS E-BIKE MODEL:
» CLASS 1: Bicycle equipped with a motor that provides assistance only when the rider is pedaling, and that ceases to provide assistance when the e-bike reaches 20mph.
» CLASS 2: Bicycle equipped with a throttle-actuated motor, that ceases to provide assistance when the e-bike reaches 20mph.
» CLASS 3: Bicycle equipped with a motor that provides assistance only when the rider is pedaling, and that ceases to provide assistance when the e-bike reaches 28mph.
In the interest of safety, local governments would be allowed to restrict, regulate or prohibit the use of e-bikes in their parks, on paths and on trails. In the absence of local ordinances, Class 1 and Class 2 e-bikes would be allowed on paths and trails; while Class 3 e-bikes would only be allowed on roadways. The e-bike class model also allows local agencies to permit e-bikes on paths or bikeways where they are traditionally not allowed, in the event that the alternative route is considered hazardous. This is a local jurisdictional decision.
The laws are changing fast, as well as where you can emtb and not. But it seems every month new places are opening trails to emtbs

some recent examples of more opened trails: WY, AZ, many bike parks, Lake Tahoe, etc....