We shouldn't forget that Ewan MacColl was brought up in Salford between the Wars. Salford, at that time, was the archetypal "Classic Slum" with high levels of poverty, unemployment, environmemtal pollution and sub-standard housing. Those of us born after the 2nd World War can only guess at what it might have been like (although I did catch a glimpse of the last of this world, in the early 70s, and could only give thanks that I wasn't brought up in it).
In the face of such adversity intelligent people were often drawn towards radical politics - and who can blame them? Unfortunately, the most radical of these political groups, the Communist party, demanded absolute discipline and obedience to the party line. It also pushed the concept of 'Socialism in One Country' ie. The Soviet Union. Many people seem to have been fooled by this and turned a blind eye to what was actually happening in the Soviet Union.
Remember, though, it wasn't just politicised working men and women who were fooled by Joe Stalin. I've just read a book about the experiences of the Polish Nation ('Rising '44: The Battle for Warsaw' by Norman Davies). The Poles put up an heroic resistance to Hitler but were ultimately smashed. Stalin just sat by and let it happen so that his armies would meet no resistance when he took over after the War. When he did take over, Polish men and women, who had fought bravely against Hitler, were rounded up, imprisoned, tortured and executed. Western governments knew very well what was going on but they, in their turn, just sat by and let it happen. They only had themselves to blame when Stalin got himself a Bomb.
I still occasionally meet old Communists who believe that Stalin was a hero and that the Allies couldn't have won the War without him. I, personally, think that it was ordinary Russian people who turned the tide and that they did it in spite of Stalin who considerably worsened their sufferings.
I don't honestly know what Ewan thought of all this towards the end of his life; I hope he had a more balanced view - but I'm sure someone will tell me different.