They have 49MM users in Japan alone, and Japanese users are famously loose with digital goods spending (their mobile gamers spend many times that of counterparts abroad).
People buy the new sticker packs so that they'll have stickers nobody else has. Other people will start getting these new sticker packs, and thus the "trendsetters" need new sticker packs in order to maintain their uniqueness. LINE is happy to keep releasing new stickers to feed this cycle.
http://static.guim.co.uk/sys-images/Technology/Pix/pictures/... http://www.theguardian.com/technology/appsblog/2012/dec/04/i...