Oooor maybe they knew that the US stranded him in Russia on purpose to try and pass them fake information. Their agents had already found out about the plot so

Oooor maybe they knew that the US stranded him in Russia on purpose to try and pass them fake information. Their agents had already found out about the plot so they decided to play along, providing a tactical advantage, by letting their American counterparts think the mission had been successful. Or it took much less effort to offer him asylum and score political points, knowing full well that the disclosures would also be damaging to 'enemies'. Or they knew he had information on them and that it would come out as well. Or maybe they already knew what he knew through other sources and didn't need his documents. Or... Or... There are many possible scenarios (some much more ridiculous than others) but it is always lots of fun to just jump to the one that is most like a spy novel. It certainly is the most entertaining no doubt!