CallMeSconessaid: Is the system basically saying "He buys keys so he doesn't need the promo vs the guy rarely buys keys - let's entice him into buying."
Yeah, that's the gist of it. Take a long enough break from buying keys so that Jagex deems you as an inactive buyer, and throws an offer your way.
CallMeSconessaid: Seems kinda unfair that people are receiving promos that aren't available to all users.
That's rich coming from someone who spends money on an unfair system, Treasure Hunter, which not everyone can afford throwing money at. If you cared about fairness, you'd want Treasure Hunter be removed, so that everyone would have a level playing field.