Well, for one, many of these companies, like Facebook, are not US companies. They're headquartered in Dublin, an EU country.

And, you don't have to abide by foreign laws as long as you never intend to go into that country or do business with people in that country. But, as many EU companies are about to find out, they're subject to US laws, most notably those pertaining to sanctions on Iran. So why shouldn't US companies be subject to EU laws?