As informed in comments section, seems different versions of Angry Birds or even other apps are free in different regions.
As per Keng,
Change your location
Star War, Season, Rio = Thai Store Free
Star War2, Angry Birds original = US Store Free
Space = France Store Free
As per Venky,
cut the rope – free in india
cut the rope exp – us
wheres my mickey- in us
pencil blade premium – us
insider pro – india
insta weather pro – india/us
I don’t know how is it happening as there is no prior announcement regarding this, but was just tipped by our reader LoRd1 that most of the Angry Birds games are free at WP store.
On doing a search it seems Angry Birds, Angry Birds star wars and Rio are indeed free. So grab the offer before it expires as it is too good to be valid for long. Check the source link at bottom of the article to check all the Angry Bird games at WP store.