This is a rather crazy line of thought once one actually starts to think about it. If I see a poisonous (say rotten) food being sold, I find it almost my duty to inform people. Somehow it's now being argued, that I should stay quiet and vote with my feet/wallet instead? If a company does something clearly distasteful and harmful to others, staying quiet and voting with your feet is not a good idea. This should be obvious to everyone. At best, you are ignoring the problem, and at worst, you are implicitly allowing the situation to continue.
If something is harming others, staying quiet or saying that "users should know better and not act like they do" is not the way to go. If the search box was labeled "ask amazon about this", then this would have been a no-brainer and no one would object to this. Its the same as labeling some food-like objects as non-edible. Canonical search however does not do this, and tricks users to send data that they believe is private.
Imagine a worker at a company, using this function to search for internal document including a string with company secrets. Whooops, now that data is at Amazon. Imagine a police worker, seaching for email with witness details. whooops, now that data is also sent to Amazon. Imagine a normal person searching for emails that includes their credit card. Whoops, gone gone. And let's not discuss things that private people honestly, truly, do not wish other people know, or for that matter, journalists.
This is not fair to the users, and RMS points this out. It might even be illegal in some if not all european countries. At the least it's something to write about and inform users about the risk involved.