First off, Paul, nice photos. I also commend you on contacting and negotiating with the zoo. Often times rules are created, but with cool heads and thoughtful planning, they need not be inflexible. After negotiations, you may come to an arrangement that would benefit both parties.
The posts stating that the animals need to be fed, protected, kept healthy, etc. are imo correct. An organization has collected animals, created naturalistic habitats and maintains the health and well being of these exotic animals for your and the public's enjoyment and education. All of this comes at a great (ongoing) cost to the organization. Benefactors and donors (as well as visitors' admission fees) offset these costs and keep the zoo open (sometimes barely) so you as a visitor and photographer can enjoy the opportunity to view and take some photos of animals you might otherwise spend thousands upon thousands of dollars tracking in their various countries of origin. Once you start making money from the efforts and goodwill of these benefactors, it doesn't seem unreasonable for the organization to realize a revenue stream.
As I mentioned before, the $250 fee may just be an arbitrary number, or starting point at which to negotiate. Previous posts suggested that some arrangement might be made to license images for the zoo's use in exchange for various concessions on the fee angle. What worries me about some posters' cries of indignation and suggestions that you circumvent the rules and sell your images on the sly, or involve lawyers, etc. is that you might make matters worse. A zoo director, if faced with an angry group of photographers threatening lawsuits may back down on images you've already created, but can very easily update the policies to exclude ANY camera with a detachable lens or tripod. Their reasons for this would simply be "health and well being of zoo visitors". Circumventing this policy may end up with you being escorted from the premises by the police. The zoo can defend this policy much easier to a judge by using the safety of the public as an excuse. What judge will take the side of the photographer, saying "Well, Ms. Zoo Director, I think the chance that an elderly patron tripping over Paul's tripod and breaking a hip, or a grade schooler being clocked in the face with a long lens as Paul tracks a bird in the aviary is well worth the risk. Therefore, I must strike down your rule prohibiting this equipment, in the interests of Paul getting some really nice shots"? It may seem unlikely that you'd cause this type of calamity, but a determined (and ticked off) zoo director could make your life very difficult.
I would just chalk it up to the cost of doing business (which it is if you're selling the photos), and do what you are doing...negotiating and entering a dialogue.
Good luck, and let us know how it turns out.
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Bill
http://www.shoeflystudios.com 
