Has the rule ever really been challenged? Since court cases tend to take years, has any player ever bothered to make the challenge and take it all the way to court? Why make a challenge that would probably last longer than most pro careers. But if no one has ever challenged the rule, then how can we say that it is definitively legal?
On a separate unrelated note, would the CFL even be relevant to the court fight? I'm not sure that the argument that one could leave the country to work would matter to a court deciding matters within the US (I'm assuming Arlen Spectre isn't involved *g*). In terms of a US case, the relevant alternatives to the NFL would be some of the semi-pro leagues and the Arena league. Though, I can only recall Eric Swann being the example of a semi-pro player who was drafted so that might not be a good example.
If there is no law directly prohibiting it, and no court has made law by ruling it illegal, then it's presumptively legal. To be definitively legal, a court would have had to bless the practice. And I guess we'll find out which way it is in the next couple of years.
This will be an interesting case. However, it would be a lot stronger if a club that wanted to draft Clarett was suing the NFL. He probably faces a pretty good legal challenge that he has no standing to sue.