Seems like all the Angry Birds Games from Rovio has gone free. So if you been holding up to a couple of them because they were priced a little more, its time you download them now. What it looks like is that Rovio is slowly turning all the older games free to keep the new consumers entertained, and convert them for new games they would launch.
Angry Birds games has always been free on Android since the launch as they make enough money from large user base. On Windows Phone, its more of a paid model which is later taken down.
The list of games are Bad Piggies, Angry Birds, Amgry Birds Epic, Angry Birds Season, Angry Birds Star Wars, Angry Birds Go and Angry Birds.
That said, Its time you grab them and keep your collection growing. You can uninstall if you are running out of space, and download anytime later when you want it.