[dozy]

> I've seen things like this before

It's actually pretty common in Android. Methinks you're an iOS user :)

> any app could register to listen to those addresses

This is core to the Android IPC design. Apps/Activities describe what they can handle (Intents, actions, filters), and I think it works pretty well. Android will ask you if you want the given app to be the default or not, and you can always un-set the default. Also, if you have a new app that can handle the given link, the next time you try to launch that link you'll be prompted to see if you want to use the new app, and your previous selected default will be highlighted as a reminder.

Also, I'm generally a believer that if an app is registering for an Intent action filter that it does not handle well/shouldn't handle at all, then users will give it a lot of bad review feedback, and apps will trend towards doing the right thing.