Yeah, there's that, and then the logistical challenges of having to turn the arena around on short notice from night to night with two sports on basically the same annual schedule are probably pretty significant. I doubt he wants to worry himself with that.
I don't think that's that big of concern, frankly. No shortage of people who know how to do that. I count 10 (Detroit will become 11) markets where the NBA and NHL share an arena. In fact, that might be one reason Bettman and Silver are teaming up on this, if they can get the number of markets where they *don't* share one (4 - the others are South Florida, NoCal, Minnesota) down to 3 from 5 that's something.