I think you are putting the onus on the wrong party.
It's easy to imagine a scenario in which Amazon might be nefarious or incompetent with my data, or in which they may share my data with another party who is. However, I don't need to demonstrate that these scenarios are likely in order for me to desire privacy: If I am merely undecided about who I want to share my data with, then the rational thing for me to do is to not share my data.
To my mind, it seems immoral to release a product that shares search data by default and without notice.
> Sometimes I feel like people are against this as a matter of philosophy...
Absolutely. Are "matters of philosophy" a poor way to inform actions?
Amazon might even knowingly share some of your shopping habits with some organization you don't want them to, but what's the worst that could happen? Is it worse than the effort you'd put into ensuring that doesn't happen?
Assume Amazon will lose your data. Assume everyone else (facebook, twitter, g+, flickr) will lose your data, give them up -- or that they are already public.
Assume someone will correlate all that data, and sell it as a black market service to allow anyone to track you, see if you're in or out of your house, assume your government will put you on a no fly list if you're buying books on military tactics, left/right wing political opinion, assume your government will become oppressive in 15 years and mark up all your kids for "special attention" based on your shopping habits today etc etc...
See also:
"The Right to Privacy by Louis Dembitz Brandeis and Samuel D. Warren" http://www.gutenberg.org/files/37368/37368-h/37368-h.htm
You are also understating this specific issue by describing the data as "shopping habits".
Sometimes I feel like people are against this as a matter of philosophy, knowing it'd be rare for it to have a negative effect on them. And I'm not saying that's bad or knocking it. I'm just trying to get a sense of why this is so distasteful. I've never seen an Amazon result when I search using the latest Ubuntu by the way. Then again, I'm only searching for applications or files on my machine. I've also never seen an ad on my desktop. I know these things exsits but don't know why I'm not seeing them.