The pool maintainer makes all network decisions. If a pool has more than 50% of the network, the maintainer can double-spend.
I see this as a good thing because it allows the network to adapt quickly. If the network is controlled by a few dozen people, software upgrades will happen within hours/days. Generators can change their code and have real effect before a new version is even released.
The members of a pool will leave if they know that the maintainer is acting inappropriately, though this is more difficult to detect on slush's pool -- puddinpop's pool is better in this regard because the miners can see the block they are working on.
He has about 9000 Gh/s.
The entire network has 129 Gh/s. The pool has ~8 Gh/s.