False advertising.
This isn't about the fitness of purpose of the iPhone. This is about false advertising. We have all seen those Siri ads where Siri answers every question instantly with an accurate answer. If Siri really was that good then it certainly could have been seen as a killer feature which could cause people to decide to buy an iPhone rather than the competition, and pay extra for it.
We all know that Siri is nowhere near as good as it appears in those ads and I'm not surprised that people feel aggrieved.
Perhaps the real problem is with the way advertisers are regulated and punished when they make blatantly false claims. In the UK we have the pathetically useless and toothless Advertising Standards Authority (ASA) who offer no effective sanctions against serial false advertisers and, therefore, are no deterrent at all.
I'm guessing regulation in the US is at least as bad, or even worse.