I imagine that arbitrage should be particularly lucrative for exchange owners, especially if they collude.
I have always assumed that BFX and stamp are arbing the book before it opens. This is standard practice in U.S. equity markets: Orders are first matched in-house, only then on exchange. Most HFT is just the attempt to approximate this at the exchange tier, in a competitive, rather than a cooperative environment. HFT arbs are present on all computerized markets that I am aware of.
I think that what you have outlined is a safe assumption. And, it confirms my biases perfectly.
That assumption, coupled with my very strong suspicion that the exchanges are trading for their own profit with users assets, and my belief that the exchange operators employ those pesky automated trade bots that keep posting fractional asks and offers simply to keep up the appearance of trading activity, are the reasons I closed my trading account on Feb 17 @ 547.
The six months of trading left me with more BTC than I began with, so it's OK. My perception of the dangers of continued trading was not OK. I'll just wait.