There's lots of systemically horrible things that can happen if you're not careful about what you allow. If you're interested why these ethical principles exist in the United States, I suggest you at least skim the Belmont Report
Of course there are. But what specifically are the harms that are going to be caused by either this research or a landing page A/B test without a click through pop up? The existence of theoretical harms for broad categories of potential research does not have a whole lot of bearing on these specific lines of research.
> If you're interested why these ethical principles exist in the United States, I suggest you at least skim the Belmont Report
I read it. It was interesting, but I don't think it's particularly relevant here. The only prong of its test that would be relevant to this experiment is "respect for persons". The idea that somehow not revealing the bug or the experiment was intrinsically harmful as a violation of a person's moral autonomy.
I don't buy that line of reasoning, and I can't really think of any valid consequentialist justification for it, and the report itself does not attempt to justify it either, as far as I can tell.