I partly agree. Free software works that way, that is: It allows this. And that's good. It's the reason why I care about it and I guess lots of other people feel the same way.
That you _can_ do things doesn't mean that it's nice or morally okay.
Looking at the numbers: Yes, I don't know what Canonical needs or not. If it makes a dent. I hope that I clearly marked this parts of my opinion as nothing but guesswork. Looking at the numbers the banshee project made public I still stand to my claim though and feel that this - so far - is not money you need to tap into as a company.
Regarding the switch: You seem to imply that this is kind of a nice thing. As if Canonical was 'allowing' banshee to be the default player. You indicate that this is a kind of 'benefit'. I'd say that the users just wanted that music player and Canonical (or, the Ubuntu project) listened [1], [2], [3]. Providing a music player of their users' choice is not a nice gesture where they help out a weird, unknown & tiny project, they are choosing a product that was available (although optionally) on their platform for a long time as a new default to suit their business needs already.
Again: In my world, they are already profiting from the banshee project, ignoring Amazon sales. Integrating Ubuntu One sales was a (fine, great, understandable) business decision. Implying that they can change a 'buy here and you make Amazon donate to Gnome' (yes, the wording is obviously not neutral) plugin, because they are nice enough to distribute the banshee product as a default media player in Ubuntu is outragous in my world.
Edit: Just stumbled upon this part of your answer: 'I think it's a bit presumptuous to assume that Canonical doesn't need the revenue from the music sales.'
This is mightily misleading and misrepresenting my post. I'm not sure if you don't know or if that is a mistake, but Ubuntu/Canonical DO (potentially) already get money from banshee users. Their own store is integrated in banshee as well [4] and the decision that they backed off from [5] was in fact that they wanted to ship only their own plugin. Which would've been fine. With that decision users by default would've been forced to
a) buy using Ubuntu One (100% to Canonical) or
b) buy elsewhere, possibly using the disabled by default Amazon extension, which might donate to the Gnome Foundation.
They changed their route and I have a hard time understanding it (mind you, again, not from a business perspective. If you like to tack the Greed Badge on your shirt, and probably you need to as a company, this is fine. If you want to have code monkeys working for you for free however, this might be the wrong way to take).
1: http://brainstorm.ubuntu.com/item/2073/
2: http://brainstorm.ubuntu.com/item/9761/
3: http://brainstorm.ubuntu.com/idea/11731/
4: http://packages.ubuntu.com/de/lucid/banshee-extension-ubuntu...
5: http://gburt.blogspot.com/2011/02/banshee-supporting-gnome-o...