The way I see it is, it's not really total immunity, it's just effective immunity because it works against everything within the scope of the basic rules. Beings outside the scope of these rules, like Shelob or Ungoliant or the god of poison, would affect dwarves, because the dwarf really has, say resist poison 20 (which is the same as immunity if nothing in the rules does more than 20 poison damage), but it can still be overcome by Shelob's poison 40, or the god of poison's 100.
So it is immunity, until the dwarf comes up against something that would feasibly affect him, at which point you give it a numerical value.
Alternatively, a dwarf's resistance to poison might improve as he levels up. A lvl 1 dwarf won't be facing Shelob, he'll be facing rattlesnakes and such. A level 1 dwarf is immune to rattlesnake venom. A level 10 dwarf is immune to Shelob's sting. A level 30 dwarf is immune to the god of poison. The DM only has to worry about it if a lvl 1 dwarf takes on Shelob.