Upgrading to expansion/playtime

So I upgraded my son's wow account to the BC expansion, and inserted his CD key into the appropriate box, and then....lo and behold, his account is still not playable. Why is this? Becuase I spent 20 dollars on an expansion for him, only to find out that I had to pay an ADDITIONAL 15 dollars for him to play that expansion. Seriously, thats [removed]. Spending 20 dollars for an expansion should entitle you to the 30 days of game play, since you paid an extra 5 dollars for the game. I realize that if your buying expansions, you obviously like the game, but come on...45 dollars for a month of game time? Whats the reasoning there? If I spend another 40 bucks and get him the wrath expansion, will that equal 55 dollars for him to play that month? Thats ludicrous.