>That's because with POS systems, you're not waiting for 6 confirmations. It's just assumed that you won't try to double spend your coins, and that no forks or orphans in the block chain happen. This is not 100% reliable - if you wanted a guaranteed, irreversable transaction immediately, well, you're out of luck. Permanent or immediate: pick one.

Actually, in absence of double spends, forks or orphans are innocent: valid transactions (w.r.t a chain) that are not part of a block will be picked up eventually by the main chain. So there's only an (expensive/complicated) double spend risk, and not a (more common) orphan block risk.