No company in any domain rolls out products globally all at once. McDonalds introduces new menu items in test markets, tv shows start as pilots, software is deployed gradually and as it’s deployed it’s usually measured and rolled back if it’s not working as expected. AB tests are hardly any different. Smart companies experiment.
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.