The developers are contributing to open source writing software under the GPL (or other work under the CC). By the terms of the licence, all this work is available outside Humble Bundle and for free. Humble Bundle's business model is to collect donations for them and make it easy for users to download and install the software. For that they take a percentage. Good business plan.
But that's not at all what I perceive Canonical is doing. They are not, as far as I know, collecting donations for distribution to Debian and developers and extracting a fee for Ubuntu's role in providing it.
Programmers contribute to open source not to make money for any entity. The GPL makes their work open and available to anyone to build upon. In short, it's a service to the community. That is their motivation.
Anyone is free to try to make money with open source in any way except selling it! The GPL makes that very clear. And it's a good thing, too. If it weren't that way, Ubuntu wouldn't exist.
I'm not accusing Canonical of trying to sell Ubuntu. It's clear the donation is optional. There is no legal problem with this as far as the GPL is concerned.
I'd just rather donate to non profits. If Canonical provides a service I want to buy, I'd be happy to pay.
I'm not against the profit motive in any way. And I'm also a big proponent of open source. I just find it odd the way it's being done here. Makes no sense to me in fact.