I understood that retailers just tore off the magazine barcode or masthead (with the date) and returned that.
Presumably they are responsible for disposing of the magazines themselves?
Yes that is so I think for those that go unsold at the newsagents shops but what about those that are at the distributors warehouse for the actual part publication, most come in by the pallet load or more and a lot would go unsold and just sit there. Some would be used for back issues for a time but after that they would be disposed of I think to make room for those new ones still coming in.
Where I worked we only used to keep a certain amount for back issues, the rest got dumped even Playboy and Penthouse etc. We even used to provide the local police station with reading material albeit unofficially. The police patrols used to come in and get stuff from our dumpsters most days, the boss did not like it but the workers turned a blind eye to it.
He actually told off one police patrol one day for doing it and not a week later got pulled over for a traffic infringement and guess who the two police officers were! It was a genuine traffic infringement as well as the boss was a shocking driver at the best of times! I only went with him once somewhere and swore then and there never to travel with him again!