The USA was concerned because they feared that they would launch them, this happened in a point in time where relationships between the two countries were not good and they didn't trust each other. Then when Russia had missiles in Cuba, which is close to the US, it caused the US to freak out about it, it's now called the Cuban missile crisis.
The United States was very much concerned about the Soviet missiles in Cuba because they was worried that they could use them against the United States.
Both the United States and the Soviet Union does not believe each other there was already a lack of trust then this happen which raised the tension between the United States and the Soviet Union.
And, when the United States came to know about the Soviet missiles in Cuba then this created serious tension between U.S and the soviet union.
