They only make the site better for you if you're on the "winning" side. If A/B tests made the site better for everyone being tested, they'd not be very useful. Unless you mean "they should make your website better for me [after the test is done]", in which case they have the same goal as Facebook's study.
The goal of the study _is_ to improve the site. Facebook wants the users to return to the site, and users who become happier when using the site do that. Facebook isn't paying its data scientists to perform studies that have no meaningful use.
I sincerely hope you're not making the case that testing a hypothesis for a scientific paper is not a valid reason to perform such an experiment, but improving a website is.
