The US has the biggest financial market. The financial market of the US affects many other countries, it's like a domino effect. This will affect the bitcoin price. Other countries can ban Bitcoin today and it won't affect the price as much as when the United States bans bitcoins. It's like the US is at the centre of things when it comes to the financial market so when something happens to the US market it affects other markets to a certain degree.
Also, the US is one of the countries with the highest holders of bitcoins so what those people do with their coins might affect bitcoin price.
People have very short term memories, but the price is much higher than what it was just one year ago, so the current short term volatility means absolutely nothing for anyone that has been holding their coins for a short amount of time, so it makes no sense to worry over market movements that over the long term will be nothing more than noise.