Why would you offer a discount to people when you can make more money by not offering a discount? To me I see the coupon as a reward to the people that know about it. Plus if it's a promotion you are probably increasing the volume and taking away the price comparison for people that like to save. Plus maybe some loyalty knowing those guys always haave coupons.
Take GoDaddy for instance. They always have promotions running. If someone just wanted to purchase a domain they would go on there and pay $12 but if they wanted to save money they could drive the price down to as low as $2.49 (in some cases). They are not losing money at that price and will even make more in the long run since they didn't lose you to namecheap and for the rest of the time you own it with them you will be paying $8-10 with the convenience of auto-renew.
But then why have coupons and just offer money off to everyone?