Yes but I don't see your objection. That's basically what google, apple and alike are by definition. The marginal benefits and the techniques used to reach them are very sound. The cost function they choose is not.
I don't understand the distinction you're trying to make. The past alternative was to hire UX testers, do focus groups, and have alpha/beta testers. It's obviously advantageous for companies to instead push this work onto the users, and the cost function they choose of course doesn't take into account what are now externalities.
There are way more details to the AB testing over. UX is just a percentage of the total product. Users are being subjects because their behavior is what you make money out of. Is it moral that's a different discussion.
The UX part is mutually beneficial, so it's at least possible to make an argument that companies doing so are benefitting users more than traditional UX design processes would. I still think it's creating externalities for users and would appreciate informed consent (something along the lines of bringing back proper versions where nobody would ever be forced to upgrade).
Optimizing to the detriment of the user (eg for engagement, conversion, whatever) is straight malevolent and I've yet to run across a moral justification for it apart from market nihilism.
