> [...] Conducting a study on unknowing testsubjects which aims to influence the subject is an ethical question. [...]
As far as I understood it you are just restating what our parent already said with:
> [...] How is this study ethically responsible? [...] They’re studying the long-term perceptual impact of advertising on test subjects who did not authorize being participants in this experiment. [...]
It's not sufficient to state that there's an ethical argument to make the argument an ethical argument. This is important, because it's the only way to productively talk about ethical concerns at all - irrespective of how one feels regarding this particular issue.
An argument based on ethics is usually founded in some set of value propositions. Often a case gets structured/applied along those propositions in the argument.
> [...] Picture for example the nocebo effect, there are only limited studies on humans, as inducing these negative effects on unknowing subjects is extremely unethical. [...]
That does appear to be the ethical standard case from clinical research. You've test subjects with a variation of intentions, knowledge and consent as well as (for the most part) clearly isolatable harm.
One would first find the obvious ethical fault in intentionally inducing the risk of harming someone and subsequently start healing/weighting through something like consent, individual and humanitarian utility, etc. (That's roughly what you described as part of an ethics board workflow.)
Now back to this case: How do we quantify harm induced by a change regarding LinkedIn ad-density? It's not clear to me that there is any risk for induced harm even if all (100%) posts visible on LinkedIn were ads.
Next question would be consent, but since I wanted to start with the strongest argument first, I'll stick with the harm.
This is why I would like to know how the ethical argument against this study works.