Having a US military base (that wasn't forcibly installed by the US military) in your country is usually great for a country's security at a fantasic price. Especially if you're a country that's not likely to have an adversarial relationship with the US.

The US isn't likely going to invade or go to war with Romania anytime soon, so having US military thousands of US troops, aircraft, and the world's most advanced missile defense systems placed there offers great security that they're paying for just by being located in a strategically useful position.