I always wondered why drag-and-drop in particular was what they chose. Why not just a "Confirm" button?
The next question is: can we execute an application directly from a zip file? Or do we need to invent something that allows for a folder structure, but is also downloaded in a single file?
Once upon a time, upon seeing a disk image open with an app icon, a certain quantity of people would choose to run the app straight off the disk image, likely not understanding the prompt to copy the app to the Applications folder.
Then, sometime later, the disk image would be unmounted (e.g., system reboot) and suddenly the app they had “installed” disappeared.
Rather than force everyone into mandatorily copying apps to the Applications folder, they added a dialog to suggest to the user where the app should go. I will say, though, I didn’t realize it applied to _everything_ outside of the Applications folder. I thought it was only for apps run off of disk images.
I think that's the individual applications themselves: as far as I know osx itself doesn't care where you run the app from (or maybe this was only added after 10.10?). You can even run the app directly from the disk image, which I often do to "try out" an app.