They're in scope because they provide a pornographic service, I don't see that is arguable. If you don't have the competence in house to follow the guidelines and need to hire expensive lawyers, then yes it's an "extreme cost", but that's not true of all businesses.
But that's the whole complaint, the overhead of compliance with the law (not necessarily the controls on protecting minors themselves but just the legal burden) is so high as to be untenable for anyone but big corps.