One of the balloons was even American made! RIP K9YO-15. The government spent ~$500,000 to shoot down a ~$20 Ham radio hobby balloon.
It's an understandable overreaction. The way I heard it explained, US defense radars are tuned to ignore all the
multitudes of weather and hobbyist balloons up there. After this high-profile incident, they naturally disabled some of these filters and chose to err on the side of caution.
Yes, it's expensive to shoot something down with a precision munition, but you wouldn't want anything less - especially if you weren't sure whether it could be hostile.
The important thing is that sensors and tactics
seem to have successfully adapted, given that we haven't heard of ongoing issues with shoot-downs of nonthreatening objects.
On a related topic, I've been wondering for a while about high-altitude drones. A light-weight craft with large enough wings could conceivably stay aloft indefinitely, just using solar power. At similar altitudes, it'd be a lot harder to see than such a massive balloon.