You see, that's why there are ethics committees at Universities.
Let's draw an analogy here. Let's say that Google decided to conduct an experiment on its Glass users, and that for a period of a week, Google decided to see if it could alter the mental state of its customers by using Glass to delete or diminish positive social interactions. Let's assume this research was conducted without any kind of consent or knowledge of the customers being experimented on. What's your immediate reaction to that? Still cool? Still harmless and just like advertising and A/B testing? Or, creepy and dangerous?
Like it or not, Facebook does have a special responsibility here. It is, quite literally, the lens through which people see their world.
...What about enticing the maximum number of people to click on your buttons for the maximum amount of time, with no purpose but capturing their eyeballs?
"Do customers prefer the green button or the blue button" is harmless. Psychological testing is NOT harmless and NOT something that anyone with a website can or should do. That is medical research - and as such is strictly controlled precisely because of the real risks and genuine human cost that it can have if conducted inexpertly.
That is a psychological test.
