When publishing third party content for Pathfinder, is the published content automatically considered to be under the OGL? I was browsing http://www.d20pfsrd.com and see tons of third party content. Is this because they are required to allow this or are these simply the third party publishers that have consented to be on the site?
No, third party content for Pathfinder is not automatically open content.
However, many third party products do contain open content in whole or in part. The way you can tell is that the product itself tells you. Any OGL product contains a reprint of the OGL license along with a section that indicates what part of the product are Open Game Content and which are not (Product Identity).
d20pfsrd reaps the portions that are Open Game Content and collates it.
You'll note that even some of the Paizo classes from some Golarion books have different names on d20pfsrd; that's because the class itself was declared open content but the name had product identity in it. Correctly identifying open content requires careful reading of the license the content's delivered with, avoid any sweeping generalization because it can get you into trouble.