The soviet union essentially allows east german citizens to leave if they want by going through

"After East Germany tightened its zonal occupation border with West Germany, the city sector border between East Berlin and West Berlin became a loophole through which detection could occur. This was closed with the erection of the Berlin Wall in 1961. Thereafter, emigration from the Eastern Bloc was effectively limited to illegal defections, ethnic emigration under bilateral agreements, and a small number of other cases."