US OPs are currently operating in a "gray" area of the law. For the time being, the vast majority of US OPs are legal and you don't need to worry about the feds knocking on your door. IOPs, on the other hand, definitely aren't legal, no matter what they may claim on their websites. If Customs intercepts a package sent to you by an IOP, however, 90% of the time you'll just receive what is commonly known as a "love letter" (seizure notice) that most people just toss.