Who owns the zoo? Private zoo no. San Diego Zoo Global owns the zoo. Public taxpayer zoo yes. Free zoos,
http://www.trekaroo.com/list/free-zoos-across-the-us but as always the agecy makes their own rules. They can say, no zoo photos. They catch you and you can be closed. Then come to the forum and cry how your account was closed without reason or notice.
Sorry Mike the copyright on the Mona Lisa ran out years ago. The Louvre can't claim they own that. But they can say no photos, no tripods, no flash.
Private ticketed events no photos. Private property no photos. Public events on public streets maybe yes.
There are no easy and simple answers, each case by it's own.
Editorial doesn't require credentials. Microstock editorial does. Have I explained it clear? Each agency makes own rules and decides what they take or not and the law is not the reason.
You can put all the editorial images on your own site, license them as editorial, 1st ammendment rights news. But might have none accepted for Micro.
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