Some quick thoughts-
1. This doesn't affect people who are paying for Prime on a monthly basis, of course. But it is my understanding that the vast majority of people are paying for Prime on an annual basis, and as alleged, changing the terms and conditions for those people would be problematic under the causes of action alleged.
2. Next, while Amazon could rightly claim that many people don't even care about the video service that are annual members, I doubt they would even think of raising this argument given the FTC lawsuit ... and the issues they are having with bundling.
3. A brief review of the T&C of prime indicates that it does not have a mandatory arbitration clause, but it does have a venue clause (King County, Washington) where this case was filed (federal court, W.D. Wash.). And while it does have a waiver of jury trial, it doesn't have a waiver of class action. Notably, it does have a provision allowing agreement changes (including any aspect of Prime membership).
While I have questions about the interplay of the underlying claims with contract, in the end I assume that this is just an attempt to leverage a class action to get some amount of money out of Amazon for the class (and, of course, the attorneys) for the change.