Many of those legacy allocations were made before ARIN came into being and
IP assignments were doled out by the InterNIC. This was also before
IANA/ICANN started allocating /8s to the various RIRs to hand out to
organizations in their respective geographic areas.
It seems to me that this nicely illustrates a major problem with the
current system. Here we have large blocks of IP space that, by their
own rules, ARIN should take back. It all sounds nice on paper, but
clearly there is a hole in the system whereby ARIN doesn't know and
apparently has no way of figuring out that the space is no longer in
use. It makes me wonder just how much space like that there is out
there artifically increasing IP scarcity. I don't know what the
solution is, but the way things currently work it seems like if you can
justify a block today, it's yours forever even if you stop actively
using it. Maybe allowing for some kind of IP market would cut down on
that type of hoarding -- you would at the very least change the type of
value those subnets have.