My opinion is that even you're a volunteer, you should listen to criticism or don't call yourself a volunteer (as this usually means that you want to achieve something for the benefit of others (at least in part), or you're doing something in cooperation with others). If you're not completely isolated then your actions have an effect on others so you should be prepared for criticism.
The kernel is only a single part of a much bigger "Linux desktop distribution". For desktop users, work on good default configurations for all software, X (or its replacements), window managers, package managers, backports of security fixes to old software, and many other things that Canonical does are just as valuable as improving the kernel itself.
This became very loud, Apple was forced to release webkit development process to public, and then Chrome happened, phones got browsers that didn't suck, every desktop toolkit got HTML renderer that rocks, this was tremendous example of open source success.
So thank you, smarty pants, a bit more effort on your part and we'll have no Chrome, no phone browsers, no nothing.
Four years later, in 2012, Kroah-Hartman complained that "Canonical uses me as a gatekeeper of what bugs get fixed in their kernel package and sent to their customers. There's so much wrong with this, I don't know where to start..."
If the Linux kernel maintainers are that upset about this then they should have used a licence that forced Canonical to contribute (code or financially), but they didn't, they made the Linux kernel 'Free Software', and that means Canonical can do what they want with it (freedom 2 in this case), so long as they are not restricting the freedom of others. Either change the licence or shut up.