The drink coupons are for beer/wine/liquor.

Southwest will offer you a free non-alcoholic drink, or you can buy an alcoholic drink for $4-5.

I believe you are generally allowed to buy a non-alcoholic drink in the airport (after security) and board with it. I've seen people get on flights with Starbucks, bottles of water, etc. I'm not sure if this is frowned upon or not...I board with a big Diet Coke every now and then and nobody ever says anything.