If it means they'll keep doing maps, as less and less zoos are doing so now, the £1 charge doesn't bother me. As others have said, it's a revenue stream and there are plenty of alternatives for those that don't want to pay for one. Often, it means the maps are far better quality too. Marwell similarly charge £1 for a map and its on great quality paper.
Banham charge a pound and Africa Alive 50p. Good to have for those who wish to use or as a souvenir at those zoos who don’t have a guide book. There are plenty of other options for those who don’t want to pay the extra charge. I prefer to use one as I don’t want to be on my phone when out and about at a zoo / an attraction.