And after the German surrender, Stalin sent all Red Army soldiers who met with and smoked cigs with, etc, Allied soldiers, into re-education camps.
Stalin may also have feared that the Americans would learn about Russian tactics and equipment.
It was really a shame. The decision to select the Sherman tank over the much heavier Sheridan was based on what was known about the German order of battle from Battle of France in 1940. Some of what the Germans learned fighting the T-34 was the need to up-armor and the effectiveness of the 88-mm anti-aircraft gun. Had the Americans known that, we probably would have deployed armor and anti-armor weapons that enjoyed the same edge over their German counterparts as the P-47, P-51 and P-38 did over the Me-109 and the FW-190.
On the flip side, Stalin my have feared that the Americans, having seen the Russian Army first hand, would develop contempt for their fighting ability, especially after Finland and disasterous start of the War for the Russians.