You're wrong, all Source based games by Valve are native (HL2, GMod, TF2 etc), SimCity in this example is also Native. No porting happening at all. StarCraft 2, WoW, to name a few others.
If its native why wasn't SimCity not just released on Mac on launch? Also the games you list are very few, as many are ported as mac's market share is so low, the game developer has better things to worry about then releasing on mac.