I'm failing to understand how balloons are different from satellites, save for altitude; those sats could be carrying payloads other than ISR as well.
Otherwise (for example) the Apollo missions would have violated USSR's airspace whenever the moon's orbit was above it's territory, which would make space unusable for everyone.
So yes, Ballons are by definition in airspace because they use lighter than air buoyancy to rise and sattelites do not.
A balloon could also be in the way for airplanes. A balloon could also drop its payload, on purpose or by accident. The recent balloons have been reported to carry a load the size of up to several buses so I guess if that falls to ground it could cause quite some damage.
Not sure satellites can shoot missiles or drop bombs that could hit earth in any effective manner.