Over 55% of GHash.io hashpower is miners who have chosen to mine on that pool voluntarily, because it is currently in their best interest to do so.
If ghash.io starts to use it's hashpower for nefarious purposes, those miners will certainly no longer perceive mining on ghash.io to be in their best interest, and will take their hashpower elsewhere.
Having 51% of the network hashpower makes it theoretically possible to do bad things, but actually doing those bad things would certainly result in losing a large part of that hashpower, thus negating the threat.
It's just like US democracy. Being the party in power gives you the theoretical power to do evil, but if the voters find out that you are doing evil, you probably won't have that power for long.
What am I missing here?