| Apply the patch, absolutely.
Crippling xpdf has nothing to do with respecting copyright. Copyright in no way legally requires us to cripple our code to prevent the user from doing anything which might violate copyright – and fair use means that there are a great many situations in which the crippling prohibits perfectly legitimate activity.
The analogy that the upstream developer draws with the GPL holds no water. The restrictions of the GPL are a legal requirement, enforceable via copyright, and respecting copyright on a GPL’d work requires respecting the GPL.
If there are legal issues with releasing an uncrippled PDF viewer in the US, then put it in non-us.