As for "giving up your rights", this contest was actually pretty photographer-friendly. It explicitly states you retain copyright for your images (unlike some others I've seen). Sure, you're granting the Zoo the right to use the entry to promote the contest or the zoo. That hardly seems odious to me and you could even argue it's for a good cause. I meant to enter the contest but didn't get around to it before the deadline passed, the fact that you had to enter via snail-mail or else fax the contest agreement just ended up being too much hassle.
About the Zoo's restriction on "commercial" use there is plenty of legal precedent to establish that selling "fine art" prints is not commercial use. The fact that money changes hands is not what matters, but rather whether the image is used for advertising or promotional uses, or in any way implies an endorsement.
Additionally, there's a very strong argument to be made that property releases are rarely if ever required for any type of use:
http://www.photoattorney.com/2006/05...revisited.html
Sure, the FW Zoo could probably sue Leena Robinson but they'd almost certainly lose and if their lawyers have any sense at all they'll know that. About the only thing they can really do is bar her from entering the zoo again. IMHO the only real mistake she made was mentioning where the image was taken in her for-sale listing (that wasn't terribly smart).