Well, the solution to this "problem" would be simple. Use Terran laws of intensity and apply them to Caelerethian magic. Or, as Silfer already said, the "potential" of the spell decays with increased range.
If I'm correctly, Terran intensity decays with the square of the increase in radius. (Thus, having effect X at 4 meters will cost 4x more energy than creating effect X at 2 meters distance).
That way, you can have a ranged spell without having the problem of things happening at the other end of the world.
Anyway, I looked up the Spell Sphere definitions, and though I'm no magic expert, I found nothing that would disable a Sphere I mage from casting a spell outside his visual range. The latter seems a strange thing to me anyway, for
1) Sight is not the only sense a person has. Hearing, smell etc. can all extend one's range of detection.
2) No Sphere II or I mage would be able to cast spells in darkness, behind him, blindfolded, or when being blind. It would effectively mean that mages of lower Spheres could be disabled by simple removing their sight, or hiding yourself from their sight.
Furthermore, I do not really understand the analogy with the supermarket. First, the point of discussion is the visibility of the target, and suddenly it is range. To me those things are quite different, and using them similarly only.
The question whether I'm able to grasp my mouse with my eyes closed is wholly different from the question if I'm able to grasp it from a hundred meters distance.
Just my two sans worth.
KR,
Theodorus