Embargoes aren't rooted in laws, though. There's nothing stopping a news outlet from breaking embargo other than the reprecussions from the other company, like not getting news early in the future.

Edit: Wow, I need some sleep. I misread the OP, and the Wikipedia article to which I linked. I'll keep the comment up for posterity, but, yes, the company itself can put limitations in place but it's incredibly difficult to enforce.