In some ways, it's just that everything gets concentrated onto the phone.
When's the last time you saw someone bring a book, newspaper, magazine, etc., to their desk job?
When's the last time you saw someone playing a radio quietly at their cube?
Now, even seeing someone browse personal sites from their work PC is an increasingly rare thing.
This all used to be a common sight. Now it's all just concentrated on someone's personal device. Managers care, first and foremost, that you show up and do your job.
Being at your desk on your phone or on the forums while doing your job is preferable to them than you slacking off in someone other way - at least in this scenario, you're there, under their thumb, ready to be utilized at a moment's notice.
Any issues that a manager has with a phone user who is doing their job is just related to optics, i.e., "I don't care that TAC is on his phone, but he's doing it so much it might be making other departments think we don't have enough to do!"