Answer:
First, because it was in the Cold War, and Russia's attempt to isolate its side from the Capitalist side was seen as normal thing, and America proud itself to not took any military action. And second, because no one could imagine what Berlin Wall would become in the following years. It was 1961. Who could think that the structure would turn into an oppression symbol?
Step-by-step explanation:
We have to understand this point in the context of the Cold War. Germany was separated into two sides, and people from both sides were trying to pass to the other. The built of a wall was seen as something normal, and acceptable. But just the fact that Russia took this decision and made a step ahead, this was to the Americans the proof that Communism was an oppressor and bad political system that doesn't' accept individual freedom.