Exactly is a constant legal dispute :) However, the FCC pretty unarguably has authority over use of radio spectra, especially when that use is malicious. https://en.wikipedia.org/wiki/FCC#Regulatory_powers_and_enfo... They get to determine who gets to use what spectrum, where, at what power, and for what purposes.
My point was more that poaching rhinos does not really happen in US territory, and the FCC is an American agency with no international authority that I am aware of.