You don't need a physical presence to be subject to another country's laws. Disobeying a judicial order would be grounds for issuing a warrant which could easily be expanded to an international warrant for the owners of the platform.

The "judicial order" in the first place violates the first amendment, which isn't binding on Greece, but is binding on the nature of any extradition order they wish to seek in the USA.

Don't think that will happen, they will probably tell their ISP's to block access t those sites.