Interest was disabled a few years ago (someone tell me if I'm wrong) because some people were pooling all of their TBT into one account and using it to "farm" interest. So they decided to remove the feature because people were abusing it, and you only got like a couple TBT from it a day (like for most people less than 1 TBT or so a day) so it wasn't really much to miss.
For now, the interest button is just a little "decor" button that currently just refreshes the page when you click it. Possibly in a year or so, or hopefully sooner, they'll figure out a way to re-add it and make it so people can't abuse the interest system. But, like I said, for now it's just a useless button xD