first of all you need to define what is meant by "good reward" 
Maybe for some users it can be "a couple of bucks" for others is much much more
by the way with free spins I don't think you can earn more than 100 dollars in a year since its a "free promotions", at most a few tens of dollars but nothing more.
But it's also logical after all, why shouldn't a site pay a user who is not profitable?
yes, because logically he can win with free spins but if he is talking about getting a jackpot or getting a huge money to earn, I think it is not possible unless you are a god and also very lucky to have it but probably no he can win too much with free spins. Maybe it is programmed to be just only spins and you cannot trigger the jackpot, if that will be the case it will be unfair to other players if OP wins the jackpot with free spin, also there are some instances they give free spins once you have reach the required number of spending. Maybe for some users it can be "a couple of bucks" for others is much much more
by the way with free spins I don't think you can earn more than 100 dollars in a year since its a "free promotions", at most a few tens of dollars but nothing more.
But it's also logical after all, why shouldn't a site pay a user who is not profitable?
They gave free spins as token sometimes for reaching some level on the site or for promotions, though like what you said it is really difficult and if there's no luck that backing you the chance is really slim, I'm quite sure that most of us here who actively playing from our favorite casino sites experienced the chance of using that free spins, maybe some got lucky and withdraw out from this but most for sure don't have that equal chance.
We use it in the hope that luck might be there, a casual spin which we really don't think of winning big but just to add additional fun while
playing inside the casino.