It's supposed to be if you haven't played in a little while.
It's a marketing ploy, if you've BEEN playing and they give everyone who's been playing a code, the players who are already the most likely to buy it will have one.
I took a break, and the code brought me back. Which means bringing in customers who have played or signed up an incentive to come back.
There's no discrimination, you're just the most likely to shell out the cash, so you don't get one. Bringing in more customers is more important than giving stuff out for free to the people who already spend here and there.