http://www.npr.org/blogs/thetwo-way/2013/01/02/168477773/how-will-president-obama-sign-the-fiscal-cliff-bill

A person’s signature to a document represents that he or she has read and agreed to the terms of the document. If an autopen signs a document, the person does not necessarily have control of the document to which he or she is agreeing. If the terms of a document are changed without that person’s knowledge and consent, is that person still responsible for the terms provided on the document? If yes, is that fair? If no, does the autopen reduce the significance of a person’s signature?