I have only a cursory level of familiarity with the John Ross novel and while I have understood that he spends a great deal of effort and many pages accurately describing such items as bike repair, ammunition reloading processes, stamp collecting etc. (which I have been told are accurate and can be quite instructive,) it must always be kept in mind that his book is a work of fiction. The protagonists (both the heroes and the villains) do and say what they do because Mr Ross' imagination dictated that they do so. I generally avoid fiction when trying to form opinions and arguments for or against any particular subject.
However, you do bring up an interesting point. I suppose the question is whether an individual feels the consequences of allowing the WorldHunt to exist are better/worse than those involved in opposing the organization. In other words, is the "devil you know better than the devil you don't?"
I would doubt you would see actual overt legal sparring over this issue. For one thing, the hunting organizations involved probably don't have the funds to support a protracted legal battle and secondly, there would be little legal ground on which to base a civil suit. It would be difficult to make a claim on actual direct harm incurred.
What is more likely is that they will actively lobby for careful scrutiny and vigorous enforcement of the group under existing veterinary and livestock laws. All of these trophy animals will have to be privately owned and classified as livestock rather than big-game animals and/or wildlife. Otherwise, they would fall under the jurisdiction of the local Fish and Game departments and laws which would require the "hunters" to have appropriate licenses and would limit the activity to the official hunting seasons. They would also otherwise run afoul of the various laws criminalizing the harrasment of wildlife, which most states have on their books. The goal of course would be to make enforcement so strict and compliance so difficult that WorldHunt could not operate and still maintain reasonable profitability.
Aside from lobbying, the other very effective technique that is being employed is the institution of a voluntary ban on the purchase and/or support of any company, product, service, etc. that chooses to sponsor the activity. This also includes mail, email, telephone, etc. campaigns directed at such sponsors to complain about their sponsorship and to ask that such support be pulled. Market forces can indeed be very powerful. Already one such sponsoring company has caved into the pressure and pulled their support. A bowhunting magazine has also refused to continue running ads for Worldhunt due to pressure from its readership. WorldHunt seems to be aware of this as they are now largely keeping their list of supporters secret - at least for the time being. At some point they will have to make the names public as companies don't just give money out for free. They expect to have their products, services, etc. advertised and lauded in return for their financial sponsorship.