Why do some canard aircraft have dihedral on their canards?
If it is a canard, the assumption of a well thought out design is doubtful. Yes, dihedral will add to directional instability and require a bigger tail, but at least in case of the glider the vertical is sized for countering adverse yaw, so the bit of canard instability is easily compensated.

My direct response would be: To lift the core of the canard wake above the wing. But from looking at the Solitaire I wonder whether that is really true. At least on the Stagger EZ you can see that the designer tried his best to separate canard and main wing vertically.

Regarding flat spins: The spin axis normally runs through the forward fuselage, so there is not much horizontal airspeed at the position of the canard. The tail will help to dampen the yawing moment much more. A bit of canard dihedral will not make a difference.

But maybe wing dihedral was found to be lacking during flight test and changing dihedral on the canard seemed to be an easier fix? That would be my best guess, at least for the Solitaire.