First, no matter how distasteful and disturbing you may find the site to be, the site explicitly states in several places it is not a format for prostitution. Thus you just have another dating site.
Yes I know this one is set up so a guy can offer a girl money, but it never states what for. Prostitution is payment for sexual services. Call girls are not illegal. If a guy wants to offer a girl a set amount of money to be by his side is that prostitution or plain idiocy on the girl's part if she says yes?
Sugar daddy is very careful not to outright promote prostitution. They just provide a space where it could happen but legally make themselves not liable. Personally, it wouldn't exist if people didn't use it so whose fault is it really? Besides, take a peek at Match or Zoosk and try and tell me it is much different.