They do this because Christmas day is the biggest day for app sales out of all the days of the year. I saw an article where an app developer said on an average day he may sell 300 copies of his app, but on Christmas day last year he sold over 1800 copies. Not a bad days work even if your app is only worth .99 cents. Also I have read another article showing that an app can make the same profit if it is sold at $4.99 or .99 cents, mainly because if you charge less you will get more consumers!
EA has a lot of great games on for cheap right now, I've bought a few myself!