I strongly agree. Since Snowden himself is presumably in Russia, the U.S. government is effectively attempting economic sanctions against him. Their method is to scare anyone that might send him money with a lawsuit.
It's possible that Macmillan paid him an advance and so he already has the money. I don't know if the feds can do much in that case.
Also, it's entirely possible that there will be ways for the publisher to never repatriate profits from sales abroad and/or translations.
This is just petty stuff from petty bureaucrats.