A Directory Server is akin to ARIN, in that it's a directory of all the nodes and their location in the network.
Suing ARIN (and services like it) because there is a website somewhere which is connected to the INTERNET is a new form of silliness not seen before. Sure, technically, ARIN do "perform a service" which "enables" a revenge site to operate, but it is so indirect, so remote from the offending party, that it is just silly.
A torrent tracker is a directory over content and distributors, and the legal liabilities has so far been limited to mostly conspiracy charges. You would have to show that there was a "agreement between persons", which require a "meeting of the minds". Doing so is sometimes possible because the torrent tracker has knowledge of the relationship of distributors and content.
Second, as a legal matter, too much indirection is typical not acceptable for accessory crimes. Common practice normally allow only one step of indirection.
ARIN lacks in both aspects. ARIN do not know who is a distributor, or what content those who are distributors might carry, and as logic implies, can't know the relationship between distributor and content. They are also several indirection away from the alleged crime. The only thing ARIN handles is location of network nodes.
Suing ARIN (and services like it) because there is a website somewhere which is connected to the INTERNET is a new form of silliness not seen before. Sure, technically, ARIN do "perform a service" which "enables" a revenge site to operate, but it is so indirect, so remote from the offending party, that it is just silly.