2.4GHz is unlicensed, but through a few different routes FCC regulations prohibit operating, distributing, marketing, etc an intentional radiator (and many type

2.4GHz is unlicensed, but through a few different routes FCC regulations prohibit operating, distributing, marketing, etc an intentional radiator (and many types of unintentional/incidental radiators) without an equipment authorization or device certification. So the pedantic answer is that you can do whatever you want in 2.4GHz but you can't do it using whatever device you want, which creates limitations of generally 1W as sort of the simplest answer.