That's true, but then the question becomes whether Sony is obligated to provide PSN access to everyone who has purchased a PS3, even when they do not want to comply with PSN's terms of use (which probably includes keeping firmware updated).
So really people have a choice: continue using linux and most of the PS3's functionality but not PSN access, or give up linux in order to get access to PSN. Since I don't think that access to PSN can be seen as a "right" created by purchasing a PS3, I don't see this as problematic.
Maybe an interesting point of comparison would be a product you purchase with a major online component, where the online component has been discontinued. Is a purchaser of Hellgate: London entitled to some right based on the inavailability of online functionality? I don't think so. So it's difficult to see why a fairly minor change to PS3 hardware which affects a minority of users would give those users a right against Sony.