There is no specific rule about keystoning out of volcano. However, admins have been trying to prevent people from tping out of volcano. The Keystone function simply is too hard to fully remove at this moment. This means it's not supposed to be in the game, but it is. That's why we handle keystoning as a bannable offence once we have given the players enough warnings for it.
Also the keystone lets you open your enderchest or private vaults, which people might take advantage of that and store their zulrah eggs and magma before leaving the volcano in case they die and lose the items.