Unlike other folders, dragging apps out of /Applications create an alias. What controls this behavior?
Sadly, the premise behind this question was flawed. I originally posted this observation:


  Inside of a test VM, I renamed /Applications to /Applications2. The renamed Applications2 folder retained the same dragging behavior as before, suggesting that the functionality is not hard-coded to a specific path.


Unfortunately, this only remains true until a restart of Finder. In other words, if I rename the /Applications to /Applications2, Applications2 will retain its special dragging behavior only until Finder is restarted! If I killall Finder, Applications2 will gain normal dragging behavior.

This also works in reverse—a user-created directory moved to /Applications will gain special dragging behavior when Finder is restarted.

So, I think it's safe to conclude that the behavior is hard coded to specific paths, and cannot be changed. I'm disappointed, but it does make sense. I do wonder if these hardcoded Finder paths are stored in a config file somewhere that could be changed... but knowing Apple, probably not.