Is it legal to require a promo code to get a special deal?

I have always been bothered by advertisers that require a promo code, often a particular word, which you must type in on the commercial web site in order to get a special offer such as a lower price or some extras. I can't imagine this is legal since it gives one set of customers something that others can't receive simply because the latter were not listening to a particular radio or TV ad. I have sometimes heard the ad on radio while driving but obviously not been able to write down the code. When I contact the business later to ask for the code, they will not give it to me, even when I do say I heard it on some radio station. This all sounds like a form of tracking and since it gives preferential treatment to some potential customers by making them jump through hoops, it sounds very discriminating and potentially an illegal practice. Does anyone have some real insight into this form of business?