Question

Why would an American care about Soviet power in Europe?

Answer 1

Americans would care the Soviet power in Europe because of the continent's influence towards economics, politics and other sociocultural aspects that deems a threat to their sovereignty. Soviet values socialism and communism which is not what America wants to pursue. 

Answer 2

The US cared about economic and politics of the Soviet Union, because it was the other big superpower in the world, the only country that could actually challenge or defy the US, and morever its main aim was to expand their communist system worlwide. Communism was dramatically opposite to the liberalist ideas on which the US has been founded, to the market economy or the Western systems of incentives based on self-interest and gaining profits.  

The URSS had already started to exercise its influence over Eastern European countries, that had already become socialist republics. The US feared that communism would suceed in the whole European continent and become the predominant system in the world, so the US started to exercise its influence over Western European countries and adopted the containment strategy to stop the spread of communism in Europe, and also all over the world.