Ironically the only way to ensure that Bitcoin miners are not secretly colluding and you don't need to trust them is by having >50% of the global hash rate yourself, thus forcing others to trust you...
I think I can explain it, let me give it a shot.
A centralized network is susceptible to attack. A decentralized network is resistant to attack.
A centralized con-artist is susceptible to attack. A decentralized con-artist network.......
...can only scam those who chose to "invest" into a dishonest scheme. Pirateat40s ponzi scam did not impact Bitcoin itself at all (and would have been far more evident in Ripple by the way) even though at some points his claimed balance must have been in the hundred thousands or did have any implications on user balances at e.g. MtGox. TradeFortress showed that you can scam people on Ripple (by scamming them... an interesting "proof of concept"), wrote ripplescam.org after that and proceeded to scam the users of his own webwallet services out of some undisclosed amount of BTC. Still this did not have an effect on anyone NOT using his "services" or falling for his scams or on Bitcoin (the system) itself.