Is there something other than just "bad form" as to why people would not want to go through an affiliate link if the ultimate purchase price for them is the same?
Yes. Doing so makes it more profitable to post worthless affiliate links in places like this, which encourages people to do so, which makes the web a noisier place.
(Note that this is not a justification for avoiding all affiliate links.)
Also it's not adding anything, very easy for us to go through to amazon and search for the book. Might be different if the posted recommended a great book that many hadn't heard of before that really added to the discussion, still though it would be better just to keep them off HN.
