Further I would say that the parent post isn't even about this. It's about protecting the consumer and yes, I would go so far as to say that if the "change" that the websites want to do has violates the rights of the user then yeah they should be restricted in their ability to do so!
Why do users need to be explicitly informed of AB tests but not about other new gradual feature roll outs?
Frankly I think when you use a web site you are giving consent for your behavior on that site to be analyzed. I wouldn’t act indignant at traditional retailers attempting to learn from my shopping behavior in their stores so that they can improve their shopping experience. That’s just how businesses work.
Why? To make a physical analogy, you're on their property. You're in their store, walking around perusing their wares, using their tools, so of course they have the complete right to watch you.
There is no way to legislate this, your only option is to raise a stink about it and hope that they'll be more transparent in the future. You can't "require" companies to tell you how they're using your data. Once you've consented to your data being collected, that's it.
Do you understand how often websites change without asking the user? Websites are constantly being updated, algorithms tweaked, features being added and taken away. You seem to be taking offense to the fact that they're providing a different experience to different subsets of the userbase? Is that what you're trying to ban? What could that possibly accomplish?
If you don't have A/B testing, then websites are just going to do it the old fashioned way: collect data, make the feature change, compare the data. What does this solve?