Because your paying for a 'free' month you cant 'use' maybe?
Look at it this way, you can download the client for free - so they cant be charging you for that can they?
But the booster is the same price as the normal game. This is due to numerous factors but the 'against' argument is that the booster would/should be cheaper as a download since existing users do not need the client (again) or a free month that cant be used on their existing account.