True, but those arbitrage opportunities will stabilize the price and make other similar opportunities less profitable. I suspect that the real reason these haven't all been ironed out
(exploited already) is because Bitcoin exchanges lack the infrastructure guarantee that transactions close when they are expected to, and thus the fee levels wash out much of the arbitrage (for now). But arbitrageurs operating outside the exchanges can use this to their advantage and probably harvest a bit of profit on the chaos for the time being.