I taught both of my horses to sidepass and gave them a horse cookie if they did it right. So sometimes they will sidepass just to try to earn a cookie! But I only reward for it when I ask for it. If they sidepass on their own, then I ask them to stop and don't give a treat.
But I think what everyone said is true, if they learn they can get a reward for it, it will be their new favorite behavior.
I still "train" using horse cookies though. I just only reward if it was something I asked them to do.