It is true that not every child will be bad with a rabbit and that it is the parents' responsibility to make sure their children treat rabbits well. However, in a situation like a photographer's studio, most of the children coming in will not have seen or handled a live bunny before. They will be hard to control unless they are well-behaved. Many parents do not care to teach their children proper manners, and many parents do not know proper animal handling techniques either. It is the parent's job to make sure their children have a proper respect for animals and handle them gently; unfortunately few parents do that job correctly and many poorly behaved, rough-handling children will come into the studio. The person running it either has to be very strict, which will include turning away paying customers and speaking harshly to customers' children, which few people will do.
The important thing to think about here is that we are not talking about the best child, the best bunny, and the best adults in every situation. We are talking about average or below average. The average US adult only reads at an 8th or 9th grade level, to give you an idea of average intelligence.