To expand: just because someone is doing something that entails risk, doesn't mean taking advantage of that is not a scam. In fact, it's the essence of most scams.
Silverstorm said, "Hardly sounds like a scam to me. Arbitrage is inherently risky."
I am having trouble interpreting that like you suggest.
My understanding is that the behavior that was originally called a scam was making it look like there was an opportunity for arbitrage that did not actually exist. That is not just someone bearing the usual risk of arbitrage (second trade happens to be unavailable), but specifically injecting yourself into that window by making sure their second trade will be unavailable by posting an offer you know can't fill.