Canonical is doing the right thing. Yes morally as well. By choosing the MIT/X11 license instead of the GPL the Banshee developer explicitly allow using Banshee in a closed-source for-profit project without giving back anything.
To start whining about moral, now that someone actually takes advantage of this right is somehow premature – in the end you had the choice how to license it, right? If you don’t like what happens change the license! Maybe a proprietary one this time, as open source obviously is not restrictive enough for you and you have to resort to “morality”.
As for me I would be perfectly happy if Canonical would simply keep 100% of the Amazon revenue – after all its their product (yes putting together the pieces makes it something new).
As a user I care most whether the product works and I use ubuntu as it works best for me. And since canonical did a great job so far providing what I want, I think the decision should be up to them whether to spend the money on shiny new icons or to give something back to the banshee developers.