AFAIK Stars pays their affiliates (Pokerstrategy included), a one-off payment for signing up a new player, whereas other sites (eg - Full Tilt), have the option for the affiliate to recieve a percentage of the rake produced by a player they sign up.
So for Stars, Pokerstrategy does not earn anything extra if you play more, so they can't afford a rake race because the money would have to come from their own pocket (as it does in the case of Stars players who place in the Top250).
That's how I understand the situation to be, I may be wrong.
Oh and yes, you can still get paid for the Top250 promotion, as that is completely seperate from the Stars platform itself.