You do have to be the kind of person who checks your data usage all the time. If you don't use much data and never hit your limit, you wouldn't check unless you knew that Pokemon Go sucked down a lot more data than you were used to.
It's... something about the way some people approach technology, I guess. Whenever I get a new app that I use while on the go, I always check the data usage stats at least once to get a feel on how much of my data plan that particular app uses. It's like... bloody natural and obvious thing to do -.-.
(And like many of those things, what's totally obvious for one is something totally nonobvious for someone else.)
Huh. I don't own a smart phone, but my 15-year-old daughter noticed at the end of day one how much data she blew on Pokemon Go.
I'm not saying it's easy, but why couldn't anyone else do what she did?