Such sites would be illegal if not sharing the header back from the source website and be banned as much as adult websites incorrectly setting this header. It’s not a real problem.
The problem is that it's easy to make, easy to deploy, easy to make money on, and a single site opens up the whole Internet. It will happen even if it's illegal.
Compare this to adult webpages setting the header. They will probably be quite willing to do so, since they want to make their money legally, and there is probably little money in serving to kids anyway. And even if a single out of thousand adult webpages refuses, it still only opens that single site.
Worrying about the latter makes no sense, because they are sort of like organized crime. People still take drugs even though they are illegal.