Well, I visited my normal resources for all things related to this and I don't see any recommendations for this type of implementation. I just think someone decided it was the best thing to do and they did it not taking into consideration the guidelines that are in place for presenting alternate content.
Also, since they are sniffing for UA, I really see no reason for that link to even be there. They could surely include it within their main navigation.
Just because someone like Amazon is doing it, doesn't mean that is the right thing to do. In fact, in this case, it isn't if you read the guidelines. That method would probably be risky for most sites. ;)
So, we have a reputable accessibility resource suggesting hidden text. We have Amazon hiding links to accessible content. I think both of these methods are in violation of the SE's guidelines and also those guidelines set forth by the authorities on the subject of accessibility.