Exactly, whichever is most effective. The aim of the law is primarily to protect against descrimination, not to blame a certain party. Both the publishers and the buyers are at fault but whichever changes get the problem fixed seems the best way to go.
Getting Facebook to disable some targeting option seems like a much easier route than going after each ad buyer. Targeting the publisher makes sense because they are in the best position to enforce policies on their platform and stop the descriminatory practice at a larger scale.
