Terms and Conditions for websites often include provisions pertaining to Copyright Law. To what extent can those Terms and Conditions, as a contract, alter standard copyright law and Fair Use? Can they be more protective than copyright protection? If the terms and conditions give permission to use some information, but then later the terms and conditions change, could that be an infringement of copyright law?
Fair use is an exception to the exclusive rights granted to the copyright holder as a defense to a claim of copyright infringement, i.e., that defense is raised when the accused infringer either doesn't have a license from the copyright holder or his activities allegedly exceed the rights granted in any license. The Terms and Conditions for a website can grant a license allowing a person to reproduce, distribute, make derivative works, or all of the above, under the conditions included in the Terms and Conditions. For instance, the Terms might allow the creation of derivative works for non-commercial use, but not for commercial uses.
Fair use provides a defense when certain types of uses might exceed the license granted in the Terms. So, you might create a derivative work that parodies the original, when the Terms didn't allow for derivative work in any circumstances. Here, the fair use doctrine curtails the scope of the copyright holder's monopoly.
As to changes in the terms, a person who makes a derivative work according the the terms at time A can likely claim their work was licensed under the Terms then in effect. They could also rely on the doctrine of estoppel to argue that they relied on the Terms then in effect to expend their labor to create the derivative work in accordance with the conditions set by the copyright holder, and that the copyright holder expected people to act in this manner. Therefore, the copyright holder would be estopped from asserting a copyright infringement claim against the derivative works if they later changed the Terms to be more restrictive. The strength of these arguments, however, would depend greatly on both the language used in the Terms and on how the work alleged to be later infringing compared to the two sets of Terms.
I am not a lawyer. but have a fair amount of experience with copyright issues. Still if this is important to you, you should get legal advice from a lawyer knowledgeable about copyright and IP law in your country.
A contract cannot change the law, but copyright law in particular provides for honoring agreements between parties, and honoring and enforcing license terms imposed by the copyright holder. A license can restrict the use of a site in various ways. For example, a coding advice site could state that its examples are for education only, and may not be reused as part of a commercial product.
Under US law, as best as I understand it, a license or contract cannot restrict Fair Use rights, as those are rights given to any user without the copyright holder's consent. However, being informed of the rights holder's objections might weigh on some of the fair use criteria. Remember that fair use is a case-by-case determination, and the precise facts may change what is and is not allowed as fair use.
Fair Use is a very specifically US legal concept. Somewhat similar concepts in the copyright laws of other countries are significantly different, and go by different names. It is NOT safe to assume that fair use concepts apply if the work is published out of the US. This is a case where the precise facts would be important, and legal advice should definitely be sought.