I've heard some people express disapproval of this practice. The con does basically get to sell the same table twice. To be honest, though, I do consider it a fair solution; the only tables that get resold belong to people that didn't show up or contact the con to claim their table (and really, why would you EVER do this as a professional?), and it gives artists an opportunity they may not normally have gotten. In the end, it's win-win.
Rock on, guys, and never stop drawing.