Here's how it's still legal...
The people who put on the PS3 3.2.1 lawsuit failed to hold forth a legal theory under which Sony was liable. Therefore, there is no case law in which a party was enjoined from doing what nVidia is now doing.
This is not to say that there is *not* a legal theory; only that the PS3 class action idiots failed to put one forth. I can think of several theories that would apply; several of them bear on the insistence these companies have on treating intellectual property as real property:
(1) An easement is a non-possessory right to use and/or enter onto the real property of another without possessing it. Sounds like a software license, doesn't it? In this particular case, the right to run the old software on the nVidia device -- or the right to run "Other OS" on a PS3 device -- would be either an implied easement (based on the practices and customs of use for a property), or an "easement by necessity", or easement by prior use.
(1)(a) The strongest claim for an implied easement in the case of a firmware update would be for persons who have had prior use of the easement (in the PS3 case, it means that you must have loaded an "other OS"; in the nVidia case, it means you must have periodically used or relied upon the features being removed).
(1)(b) The next strongest claim for an implied easement would be the intent of the parties; what was the intent nVidia had, when they shipped the features being removed in the update? What was the intent of the person purchasing the device, prior to the removal of the feature, and their expectation of non-removal, if any? Similarly, in the PS3 case, what was the intent of Sony in offering "Other OS"? Was it to drive sales, such that they received benefit from it? What was the intent of the person when they purchased the PS3? Was it only to run "Other OS" (in which case, not updating the firmware is not an issue), or was it use of both the "Other OS" feature *and* the features that would be removed as a result of *not* updating the firmware?
(1)(c) An Easement by necessity could be established in the PS3 case for "Other OS"; like a land-locked parcel without access to a public way, necessity may be established if there was no other way to reach the parcel *and* there was some original intent to provide access to the parcel. This argument would only be likely to be usable by someone who had in fact used "Other OS" on a periodic or regular basis. Given that I do not have the entire laundry list of features that currently exist which will and/or will not be lost when the nVidia update is declined, I can't state for a certainty one way or another whether this could apply in the nVidia case as well.
(1)(d) An Easement by prior use. You would be unlikely to be able to establish this in the PS3 or nVidia cases, given that three of the five elements to establish such an easement are not present: (i) common ownership, (ii) severance, (iii) continued use after severance. It bears mentioning, however, because the threshold for the definition of "necessity" is more lenient than in (1)(c), and a clever lawyer could /potentially/ construct an argument.
OK, what other theories are there?
(2) "Intentionally blocked view"; if your neighbor intentionally and with forethought, built a fence, or plants trees/bamboo that subsequently block your view, and thereby devalued your property or your enjoyment thereof; the legal term for this varies, but it's often called a "spite fence".
(2)(a) The "spite fence" argument, is clearly applicable in the Sony PS3 case, since you would lose access to existing features of the device should you *not* install the firmware update, and lose access to existing features if you *do* install the update could likely be easily construed by the court, especially with a little prompting as "malicious intent" -- a key factor required for judgement on your behalf. Again, I don't know if you could make an "either or" case with the nVidia update -- bu I expect you could, for people who bought the device after the update announcement, before the update was released, and the terms on the loss of the games revealed, on the basis of the forward compatibility.
All in all: the PS3 class action guys should have done a better job.