The reasoning remains the same as with earlier smartphone bans (such as those at Dave Chappelle's events). It's partly about ensuring that fans are actually enjoying the concert rather than trying to record it all. And it might just drive ticket sales -- you may be more likely to attend if you can't just rely on a friend's Instagram feed to see what you missed. There's also the side benefit of keeping any missteps out of the limelight, although artists usually reserve that for warm-up gigs or one-off shows.
The difference, of course, is that most such bans still keep pros on hand to preserve some moments and create an ideal (if sterile) version of what happened. Here, Lamar is more of an egalitarian: if you can't capture that live take on "Humble," neither can the local promoters or news outlets. While we wouldn't expect most artists to go this route (lesser-known musicians often thrive on the exposure), you might see it happen more often with artist big enough that a total camera ban only adds to their mystique.
Update: Lamar's representatives informed Billboard that the Guardian report "isn't true." You can still use your phone at his concerts if you'd like.