Would it really be lower on the bot net in the majority of cases? I'd imagine that real users probably wouldn't want to have their entire cpu spent on this.
Not only that, real users actually want to use the service, not overload it. A real user might only make one request a second. A botnet device is trying to make a thousand requests per second to overload the server. Even if they each have the same CPU as a normal user, now each node in the botnet can only make as many requests per second as a user or the user can outbid them.