"Of course most importantly, every time you purchase something from Amazon, which most of us were doing anyway, Canonical gets a cut from Amazon's end. If you like your free OS, surely that is an easy, free, zero-effort and non-obtrusive way of helping pay for it?"
You make a good argument IMO but there are also flaws within your reasoning. Because lets face it: without Debian there wouldn't be an Ubuntu. Most of the real work isn't done by Canonical at all, but with all the volunteers which maintain the Debian packages, which also eventually find their way into Ubuntu.
As such your argument could also be easily turned around: how much of Canonical's cut finds its way back into Debian? If Canonical likes their free OS so much to build a whole company on top of it, surely its also an easy zero-effort to help pay for it?
Yet somehow I don't see that happening.
With that in mind I think people have every right to complain or share their discomfort. The OS isn't free afterall; it has a pricetag attached. You either pay through advertisement, or invest some of your own time to de-install the whole lot, time costs money too you know.
IMO there's more to this than merely "helping pay for the OS".