Bob is probably right on the money, however I can think of a couple of factors that make the connection to the water line different from the connection to the CEE or to a ground rod.
1) The connection to the water line serves as both the grounding electrode conductor to the underground portion of the pipe _and_ as the bonding conductor (thing EGC) for the interior portion of the piping.
2) In an urban area with metal underground water pipe, and with multiple services that share the same service transformer, the metal water pipe becomes a metallic path between multiple neutral-ground bonds. This means that the water pipe and its 'GEC' may function to carry full neutral current if the proper neutral is lost.
IMHO if 1) is the reason, then the code could reasonably permit running a small conductor to within 5 feet of the water pipe entrance, and a larger conductor to a closer location, rather than simply requiring the large conductor only. If 2) is the reason, then this really shouldn't apply to large installations where a transformer feeds a single customer.
But I don't know the real reason. I'm just guessing.
-Jon