I think this "problem" is mostly bogus, and here's why. Doesn't it seem like it would be extremely rare for this feature to actually cause anyone a noticeable dent in their data usage and phone bill? Think about it for a minute. WiFi assist only activates for apps running in the foreground, and according to Apple, "...doesn’t activate with some third-party apps that stream audio or video, or download attachments, like an email app, as they might use large amounts of data." So for starters, it's pretty limited as far as what apps will even use the feature, and under what conditions it will kick in.
For most people, it's only going to activate when you're coming or going from a WiFi hotspot, while playing streaming audio or something like that. In that circumstance, cellular data usage should be negligible, because you were going to switch over anyway. This feature just did it more smoothly a few seconds sooner.
The only scenario I can imagine where someone might be surprised by this is if they're actively using a data-intensive app while remaining stationary in a location that has a weak wifi signal. But here's the thing about that. Except in circumstances where something has just changed about that wifi signal, that location would normally have weak signal, and the user would be accustomed to not being able to use data-intensive apps at that location. So the circumstances where this would create a noticeable problem would be someone using, for extended periods of time, a data-intensive native app, in the foreground, at a location that usually has crappy wifi, where data-intensive apps usually don't work well, but now suddenly the video comes in flawlessly in HD.
All that has to happen while the user thinks to use that video app in a spot where they couldn't do so before, and isn't in the least bit curious about the change, and doesn't notice that their status bar says "LTE," but does notice their cellular bill went up suddenly, and they actually are able to figure out why and isolate the problem to this one feature. As you tick off each of these accumulating conditional circumstances, the probability that this actually happened to someone goes down. With a few million people using iOS 9, the 'law of large numbers' suggests there could be a small handful of people for whom this scenario has played out. I'm not sure that that number would rise to a level justifying a class-action, though.
But wait. There's more. The other thing that makes me think the problem is mostly bogus is the fact that the alarm about this started making the rounds on social media within days of the initial release of iOS 9 to the public. The only people who could have lived through a low-probability scenario like the one described above long enough to have gotten that surprising phone bill and figured it all out are people who had been using beta versions of iOS 9. The probability of this sort of surprise happening to someone trying out the beta version is even lower, though, because that person is already cued to look for changes and wonder why they're suddenly able to play HD video where they couldn't before. That person's going to notice their phone says "LTE" before they binge-watch several seasons of "House of Cards" on their iPhone in that one room of the house where they couldn't get good wifi signal before.
No, Occam's Razor says to look for the simplest explanation. The simplest explanation would be that internet trolls came up with a trumped-up thing to try to create negative buzz about iOS 9 and the iPhone 6S release. That could be for reasons of general hater spitefulness inherent in trolling, or because Apple competitors intentionally planted the seed to try to inhibit Apple's sales of the 6S. Then some enterprising lawyers picked up on the trolls' output and dreamed up a class-action fishing expedition.
