Yes, that is kind of a catch-22 for them. You can't be convicted (in the US) of copyright infringement for downloading a dummy file (something that isn't copyrighted). You could be convicted of attempted copyright infringement but no such law exists. On the other hand, if they use the real file (actual content) then they have another problem. Since these are agents of the copyright owners and they are the ones making the file available for free, they can't then go and sue people for taking it. It would be like putting a sign on your car "Take me, I am FREE" and leaving the keys in the ignition and then trying to arrest the person that took it.
That is why downloaders are not pursued in the US.