I don't know if this thing has any kind of "contract/agreement" how it should be used between Canonical and "content provider" but my thoughts on the matter:
1.) Put all available "service provider" in Unity that you think can bring some income and users are interested.
2.) Tell users why you did this. For income and to bring services closer to them.
3.) In Filters area label every singe "content provider" with button (ON/OFF) and setting should stick. If ON then show results if OFF search data must not be sent to "content provider".
This could work and when Ubuntu user is interested in Amazon stuff it enables "content provider" named Amazon and finds what he/she wants and buy it and then if he/she does not wish to send any more search data to "content provider" named Amazon can turn "content provider" named Amazon to OFF.
If this will not work like this i imagine two things will happen:
-After Ubuntu install "content provider" named Amazon will be UNINSTALLED.
-If it will be "hard coded" in Unity then i guess a lot of negative press against Ubuntu will be made.
Give user total control and educate him/her how to use it to benefit both sides!