The photograph is probably incorrectly labelled; surely this is a "walk-in" exhibit rather than "walk-through", given that not all the visitors who enter are certain to leave.
Did they put all the snakes in the circular enclosure and just hope they'd stay put?!
@Damson - Thank you for the info. Its a cool idea but on the other hand its kinda creepy. Do you know if there is still a walkthrough anaconda exhibit at (I think) the Paris Zoo.