You plant an apple tree, expecting to enjoy all the fruit. It grows until the branches extend over my fence. I wait for the apples to fall and I gather them and eat them. You've lost them. Have I stolen from you?
No. Although local law would have to be consulted to see if you could take the apples off the tree or not.
If I put the apples in a bushel basket and a high wind slid the basket into your yard, you'd probably be stealing to take and eat them.
But the difference is the apple fell into your yard, and you can take it and eat it as you see fit.
My signal may be in your house, but when you start sending your ip packets, they are coming through my house.