I am going to assume you live in the USA and seem to think the world ends there.
In the EU the consumer protection laws make warranty a matter between the *seller* and the consumer, the manufacturer is not a party in warranty issues, so whatever Apple claims about warranty is null and void here. Added to that, a seller is not allowed to 'void warranty' if nothing physical was changed (like any change in software).
So the claim that warranty will be voided does not apply in all cases as you make it out to be. I have a jailbroken iPhone that I am currently restoring to 5.1.1 because i I have every right to do so, and this will not affect my 2 year warranty in any way.
And if Apple provides a public forum for help with their devices, but censors completely legal solutions because of commercial considerations (as they seem to do) they are in violation of at least a dozen different EU laws and regulations since they impede legal use of their products and they have the choice of not providing the public forum at all, have a specific EU forum or block it for access from the EU entirely.
But as always, their pockets are deep enough that no consumer is stupid enough to take them to court over it, so they can just continue with whatever they like. But some consumers will recognize this behaviour and just never buy Apple again (as I will after I receive my Lumia 920), and once the new is faded from their products more and more people will follow.
There is no (technical) reason to disallow downgrades that helps the consumer, just the obvious reason: pushing their customers onto what Apple wants (for instance being competitive in the maps business) rather than what the customer wants and chooses (for instance mature maps). The Apple consumer is forced (and I would say duped) into accepting an inferior product to the detriment of their own experience with their (expensive) product, so they have no choice but to help Apple mature their maps and compete with Google.