Converters are legal.
The federal government directed that it was legal for consumers to own their own phone. It is legal for you to purchase and it is legal for you to own. When you buy a cable box it is your responsibility to inform the cable TV company that you are accessing their signal. It is illegal to receive their transmission without paying for it. Some cable box sellers provide systems that hardly provide any benefit to the purchaser where the quality of reception is poor, without recourse.