Polypropylene is a pretty soft plastic, and the only one that allows for what is called a "living hinge", which means it can be made from a single mold. With that in mind, yes, they all have some deflection in the larger faces (front and back covers). There's really no other option as far as plastic selection goes. The only thing that the manufacturer could change are the additives used and the mixture of the raw materials in order to enhance any number of specific qualities (shatter resistance, rigidity, density, etc.)
While not the highest quality PP, it's pretty decent, especially considering the price.