Rain falling on an area is part of the ecology of the region and is accounted in the water distributions of the local governments. Rain replenishes lakes, creeks and rivers that supply certain regions of the country. If we didn't have laws like this, ridiculous or confusing as they may seem, we'd revert to the days of the old west when ranchers damned creeks and riverbeds on their property (some of those Texas ranches back in the day were tens of thousands of acres large) causing small towns and smaller homesteads down stream to dry up. Range wars were started over this sort of thing.
And in regions where there is heavy snowfall, the 'powers that be' use this precipitation to help determine how to distribute their water.
By the way, the air space over your property doesn't really belong to you either; it is governed by the FAA (or some other such aeronautical organization.). When you buy land, though, depending on the real estate contract and/or the local laws, you do own the soil and the minerals on it. That's how many Texans and Okies became rich oil millionaires back in the day!
Jan in VA