Let me try to explain the rationale for the portfolio. These models are not freelance models. They do this for a living and have a certain standard and fame where they come from. And like cheeseme commented, what if a rich pervert pays for a shoot and takes a bunch of undesirable pictures that are spread on the internet? It might affect the models' reputation and career negatively.
It is only natural that they want to protect themselves and their career. Try to put yourselves in the ladies' shoes
As for the "soliciting" comment, I think its uncalled for and unfair. They have plenty of events around the world (different parts of the states, Japan, etc). They do not need to "solicit" any photoshoots.
OK, now for the question about how the shoot is conducted, the theme, location, etc. I could force a theme on you. Easy and no hassle for me. I left it open-ended because you, as a photographer, can let your creative juices flow. Isn't this what being a photographer is all about? Don't you want to do your own theme once in a while? YOU are in control of where you want the shoot to be done; YOU are in control of how to pose the model; YOU control the elements of the shoot. I don't know about you, but I personally feel its a good idea