I used to work for a (now defunct) newspaper & magazine wholesaler.
Unsold newspapers/magazines were returned, scanned in and credited back to the outlet.
There was a office team managing this, so say a shop had 5x copies of a magazine and only sold one, then they might only be sent 3 the next month.
Obviously if they didn’t sell any then they might stop receiving completely, and their allowance moved to another outlet, hence why you might not see that mag on the shelves in your local shop as much.
It was crazy when all the unsold magazines/newspapers came back and they were just chucked in a skip.
Company policy was that it was stealing if you were caught taking anything off the premises.
During my two years there i half inched a copy of the Dirt100 and that was it (even then i asked for permission from the returns manager!)
I think the new Sender/Cranked/Shredder magazines are the way forward, not everyone wants to digest their media via a computer/tablet/phone!