That was just because the Russian's didn't trust "their" Germans as much as the Americans trusted "theirs". Operation Osoaviakhim was mostly about knowledge extraction, not about cooperation. The other difference was that the "Russian Germans" didn't go to Russia on their own will (https://en.wikipedia.org/wiki/Operation_Osoaviakhim).

Once the knowledge was "extracted" the Germans were allowed to return though (good thing Stalin was already dead at that time).

Former Junkers engineer Brunolf Baade then went on to build the first and only German jet airliner for East Germany - which then of course was cancelled as soon as it was ready to enter production: https://en.wikipedia.org/wiki/Baade_152).