Why is the US paranoid over a balloon?

DaveofSE3

Dozens of meteorological balloons are launched every day in all parts of the world. They are supposed to burst and return to earth when they reach a certain height but sometimes they don't, they simply continue to rise and get caught in jet stream winds. They are not a great spy platform as navigational control is minimal, they are easy to detect (the balloons expand with altitude) and they have no defense against being shot down. If they did gather intelligence, it would not be easy to transmit this back to their owners.