Actually, no. You are putting the cart before the horse. What actually happened is that they tweaked the ranking algorithm, and then measured a minuscule effect in a particular scoring algorithm (in this case counting certain types of words used in future posts).

So the nature of the scoring algorithm (counting emotional words) used to measure the impact of a change makes deploying the A/B test suddenly unethical?

Most advertising is specifically designed to alter someone's emotional mental state, and plenty of that is in a negative direction. Would you also outlaw advertising? Why should advertising get a free pass and not well-controlled psych testing? What about signs warning you not to infringe on [random local law] under threat of penalty? They create a sense of oppression. Should they be forbidden?