I believe the argument being made is more on the user's magnitude of engagement. Users interact with facebook on a very close level - there's a level of emotional intimacy that users have with facebook the platform that they don't have with the corner bakery. To draw an analogy, imagine if a bank suddenly decided to run an experiment to tell people that they had lost all their money and that there was no recourse. Wouldn't that be a pretty fucked up thing for a bank to do?