Copyright is copyright. Note that TPF FAQ's supercede United States Copyright doctrine regarding 'Fair Use'.
Many (Most?) of those meme's you are referring to are derivatives likely made without permission of the copyright owners, and may very well constitute an infringement in and of themselves.
http://www.copyright.gov/help/faq/faq-fairuse.html#howmuch
How much do I have to change in order to claim copyright in someone else's work?
Only the owner of copyright in a work has the right to prepare, or to authorize someone else to create, a new version of that work. Accordingly, you cannot claim copyright to another's work, no matter how much you change it, unless you have the owner's consent. See
Circular 14,
Copyright Registration for Derivative Works.
We can post
links to someone else's images, artwork, books, and other copyrighted intellectual property all day long and not violate any TPF FAQ's or copyright laws (US or International).
We can post copyrighted photos all day long too,
if we have permission from the copyright holder that we are able to present to TPF Staff, which basically means permission in writing on letter head or a copy of a valid use license from the copyright owner.
Here is the exact TPF FAQ in question, and all of TPF's FAQs are found here -
Photography Forum & Digital Photography Forum FAQ
* You agree to only post images and/or other material to which you have exclusive copyright, or permission from the copyright holder that you are able to present to TPF Staff. Under no circumstances will any instance of copyright infringement be tolerated.
Note: My added emphasis in the quote.
If you want to explore why all photography web sites have essentially the same restrictions for the same legal reasons, read the
Digital Millennium Copyright Act and
Online Copyright Infringement Liability Limitation Act statues.
I will gladly answer any further questions you may have, or refer you to further reading or other information, like the link for US Copyright law-
www.copyright.gov.