PoS security mechanism is that of a private enterprise.
PoS = PoW with one level of indirection removed.
Instead of
Dollars -> Mining rig -> Cryptocoins
PoS offers
Dollars -> Cryptocoins
Yes, but with PoW, you need to keep consuming
an external resource, so the two aren't exactly
comparable. I'm not going to debate the implications
or pros and cons, as its all been said before, just
pointing that out.