I'm pretty sure ChuckD's right on this, as are you, in a way.
The opening line is set by a Vegas expert, but they don't give a rat's ass about the actual NBA, they only care about motivating people to make bets (for obvious reasons). The Spurs are underdogs because that's the way the line had to be set to get the money flowing.
The line will change towards the Spurs if the gamblers start betting on the Spurs with the current point spread the way it is, but these guys are experts in anticipating people's betting habits. If the money comes in evenly, the line won't change. If it's heavily flowing in one direction, then the line will move to start getting a more even flow.