the exchange price of a bitcoin. is defined by supply and demand.
it can therefor not be pegged to anything.
example:
you do make a fixed-price exchange. where one can buy 1btc for 10usd.
now if the price on another exchange goes to 13usd per btc.
people will put alot of usd into your exchange, and you will run dry of btc(and your peg is GONE)
sorry mate.
If everyone is honest merchants and the market is enough big, then maybe supply and demand will decide a reasonable price, but for a small currency like BTC, it will take only one hedge fund to push the price to 100+$ and cash out, finally short it to 1$ from there
In a pegged system, the exchange rate is always decided realtime by a basket currency/commodities, so basically all the exchange will have the same rate published.
Of course, if a hedge fund manager put 10 million dollar to buy as much BTC as possible, then exchange will have to provide that amount of BTC to him, since almost everyone is hoarding BTC, exchange simply could not find that much BTC that he can buy, then there will be a liquidity problem
So, for a pegged currency, the exchange should have enough reserve to damp the speculative behavior and keep the promised exchange rate, this role normally is carried out by a central bank
But anyway, with a pegged exchange rate, the worst result will only be liquidity problem, the exchange rate will always keep the same, this will in a large degree discourage the speculation
