Does the “sell” part matter - like is it simply that the sale to minors can be regulated? If it is free isn’t it just transmission of information?

No, it's still illegal in most places to knowingly distribute pornography to minors - the law in my state starts;

> A person who sells, gives away or in any way furnishes to a person under the age of 18 years a book, pamphlet, or other printed paper or other thing, containing obscene language, or obscene prints, pictures, figures or descriptions tending to corrupt the morals of youth