It's private property. A good parallel would be a burglar that dropped his iPhone when he was previously in your home. Keeping the iPhone would be theft?
See IJ Reilly's post here: #71 - drone flights over Apple's campus will be illegal come summer, so it won't be an issue for them after the exception expires. Makes you wonder if they didn't think of that when they were scouting locations for a new campus.
Corporations have rights similar to individuals...and in the case of tech companies, they have the right to protect against intellectual property theft. With the advances we're seeing with consumer drones, I could see people trying to fly up to the windows at Apple Park, trying to get glimpses of whiteboards inside, or in the case of Jony's design team, model mock-ups.