From my personal experience, there’s no way to monitor this short of closing down all computers, cellphones, iPads, etc. From the lectern, there are, however, some indicators of what’s going on. For example, when students are leaning over to see what’s on another student’s screen, or when suddenly the screens start talking aloud.
I do think we’re in an era of mass distraction in large lectures.
I find this quite hilarious. Seems to be a way to subtly shame/remind distracted students to pay attention. I must say though, that I am sometimes guilty of some of these as well (if I am already familiar with the material in lecture, I will often spend that time doing homework on my laptop).
I probably wouldn’t care as a prof but I would make an effort to bury the hidden gems during class . . . and if you didn’t unearth enough of them you would not do so well on the exam. 
I had a law school professor who went out of his way to ask exam questions from footnotes in cases. 
I teach math. If I see a kid doing too much typing on his Ipad, the odds are good he’s not following along.
If I was the instructor and the students who are goofing off in such a manner weren’t causing a distraction for other students, I’d ignore them with the idea that if they’re old enough to attend college, they’re old enough to take responsibility for whatever consequences come from it.
Videotaping them would only be utilized if they come in to complain of the failing/mediocre exam/final grades as a form of “What in the heck did you expect when you goof off like that?!!”
I think the difference is today the colleges are held responsible for the success of the students.BITD if you failed it was just on you.