I think I would excuse this behavior in large cities, or when you feel overexposed to stimulation. I mean, it DOES take a ton of effort to be attentive to every single human that you encounter, whether it's in an mundane setting like a subway/shopping mall, or at work. Especially if you live in a huge city like SF, or NYC.

If you don't distract yourself once in awhile, by immersing yourself with something like a smartphone, I imagine it's very easy to get drained out with so much stimulation.

I mean, if we lived in the caveman age, or even the medieval ages where we encountered the same 20 or so people every single day, then yes, there's no need for the smartphone, but in the modern day, I think it's almost necessary to survive mentally. I look at it this way: Humans were not meant to live in highly populated areas where they interact with a thousand strangers every single day.