They also restrict your ability to copy links and text in apps, so that you can't open things in a non-walled app browser. This I believe is why sites like Twitter also uses URL conversion... There is a wild variety of ways in which they can limit where those URLs go, and I've noticed sometimes it even makes externally pointing links not work properly (Which can be turned on and off at will by the link service owner).

Those URLs also mask origination when they point to other sites, so that site logs don't provide any real specific data on where traffic to them is coming from.

The most Internet/user hostile era ever is probably going on right now. Will be interesting to see where this all goes.