OK. Example time. I use my phone to synch with my gopro as a view finder and controller, through Wifi. Directly, via a gopro app, with no internet, or router for that matter, required. I don't see how this has to be any different. All of the hardware is already present to transmit and receive data in virtually everyone's gaming setup, whether it be PC or console. Software would be the only hindrance, and since Bethesda is coding the game and the app, it's within their power. Even if it can't access the console or PC wireless adapters directly, if you're using them at all, it's because you have a WiFi router.
Now. Some people won't have Wifi routers or (maybe?) phones. But it's moot. I don't see spending possibly hours at a time sending and recieving data (and RX/TX will happen, whether in use or not, so long as it's synched) on your carriers 3 or 4G wireless network as working out well in regards to latency, connectivity or cost. I am guessing there is a USB option for those people.
The GTA app has exclusive content, which is not part of the base game, but is accessed on a server. That's the difference I am pointing out. The content we are getting in the pip boy app is (as far as anyone is aware) is the same and works directly with local base game content.
I mentioned the upcoming multiplayer, not because the app has anything to do with it now, but to illustrate that Rockstar is going the route of online content- That server considerations have been made. Even then, look at the complaints above about how impractical it is. Going through the internet is a poor idea, for something that is this simple.
There is also the fact that it has been stated that you do not need to be online to play the game.
I suppose that could be shady wording that would need an asterisk in print saying "except for pip boy app use"
But, generally if you're advertising that you don't need online connectivity during play, all features should be included.