While photographers can take images of almost anything from almost anywhere, that does not mean we can publish those images. Taking a photograph and publishing a photograph are completely different issues. This is where copyright, privacy, defamation, commercial use, and fair use come into play.
While you do not need a model release to take an image of kids playing at the park, you do need a model release to commercially publish that image. An exception to this rule is news photographs. Traditionally, newspapers have not needed a release for news article photographs. However, due to the increasing frequency of lawsuits, some news organizations are beginning to get model releases as a matter of extra protection.
Andrew Kantor has an excellent summary of what you can and can not publish on his website.

