Can you provide examples? I'm having trouble thinking of anything that is based on frequency alone. Laser light can be illegal if it is beamed towards aircraft. Radio waves can be illegal if they are transmitted with high enough energy to disrupt a licensed frequency, or if they carry information that is not compatible with the frequencies' license.. but otherwise they are still legal. I can have an FM transmitter in my car to play my phone legally because it is too low power to cause interference.
What frequency of light is illegal just in itself, regardless of power, information content, or target?