> If the objective of the traffic experiment had been to deliberately get a certain group of cars stuck in a traffic jam, this would absolutely be unethical.

A closer analogue would be a traffic experiment designed to gauge the emotional effects of a particular route. That's an important difference.

The article itself says the experiment was designed to look for evidence of emotional contagion, which is quite different from "it was designed to make people sad".

Also, in another thread it's pointed out that the effect sizes from this study were extremely small - something like 0.3% more negative words were used by ~150k people. The effect is said to be on the same scale as any minor UI change, like a size/color change of the "like" button. So it's hard to see this as anything other than folks looking for a reason to get outraged.