why is NA to DAD NS sent to all-nodes address?

I encountered one quick question when I reread the spec...the neighbor
discovery specification (1970, 2461, and 2461bis) specifies the
responding NA to an NS for DAD (identified by the source address being
::) must be sent to all-nodes multicast address:

   If the source of the solicitation is the unspecified address, the
   node MUST set the Solicited flag to zero and multicast the
   advertisement to the all-nodes address.
(section 7.2.4 of 2461bis-05)

Does anyone know why this must be sent to the all-nodes address?  I
mean, why can't this be a solicited-node multicast address for the
target address?  Since the sender of the NS joins both the group
addresses according to the DAD specification, it seems to me that
either one should work fine.  Is there any special reason for the
address selection, or is this just an arbitrary choice?

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					jinmei@isl.rdc.toshiba.co.jp

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