They're not subject to all of the laws at the same time. They're subject to the laws that apply to each user's country in that user's case. If user A engages in an interaction with user B that is illegal for both parties in user B's country but legal in user A's country, only user B could be affected, assuming the interaction gets noticed. It's similar to how if you shop on an online store like Amazon or Etsy, they have to charge your local taxes and not the seller's taxes.