For a while, lots of people have been using all manner third party consumables in name company products, then claiming for damages under warranty. Often it cannot be definitively proven (but is flippin' obvious to anyone with half a brain) that they've done so, therefore the name company has to shell out. They're not always big name, but usually one with a good rep, at least.
Most have something in the warranty about not supporting someone else's product used with theirs, as they cannot guarantee compatability, quality, suitability, etc, but it comes down to how hard it is to prove.
Given the difficulty, I don't see a problem with this any more than I see a problem with OCUK refusing to pay out if you buy an 8-Pack Fancy Build from them, and then blow it up by sticking in a crappy PSU that you bought from ** no competitor hinting **
Digital enforcement of this warranty might be dodgy ground, but I see no problem with it in principle.
This one-man witch-hunt and list-of-demands email that guy put together, though, just sounds like he's stirring things up for the sake of it and making himself out to be a complete nutter.
It's not like HP lied about it or snuck it in secretly. They told everyone about it.
Moreover, his comparison to a toaster (and the linked article's microwave) is stupid - Toasters and ovens are designed to work with generic products, whereas HP is designed to work with HP stuff specifically.
I could be wrong, but I don't think a single car manufacturer/dealer would support any of it's warranties or guarantees if you slapped a load of aftermarket mods on and then took it back to claim. HP obviously just got tired of dealing with too many people buying cheap crap and then whining about their broken machines.... kinda like how our customer centre gets tired of people complaining because they can't flush cement powder down the toilet...!!