The keepers lure the snake into a slip box behind the terrarium if they have to clean the terrarium.And if not so many or no plants were in the exhibit,some zoofriends would get excited certainly exactly about that.....
So be lucky, some zoos doing really the best for their animals....
You're thinking of Green Mambas and Boomslangs. Black Mambas are predominantly ground dwellers and, like mstickmanp said, inhabit savanna and open wooded areas.
My problem with the cage is it looks too small for a snake that averages 8 foot when adult, and often more than 10 feet. Of course, there's not a lot in the picture to give a scale to the size of the cage.