The last thing capitalist parties like the Stalinists wanted was socialists or communists as members. What they wanted was obedience. The buggers conned some wonderful people back then, fair play - I remember how baffled they all were when it all came apart in Hungary. Then again - look at Hungary NOW!
Which is why many of the communists, and almost all the socialists fled the Russian sector of post-WWII Germany. Stalin was not interested in all the different shades of socialism.
There is a fake quote of Stalin about socialists being "useful idiots." Nothing could be further from his thinking. He believed capitalists would lead to failure, so were the useful ones. The real enemy of Stalin was socialists who could keep their systems running. I am not saying I agree with Stalin, but those were his views.