You don't need to buy it, even though there's a Steam check. Here's how I did it. I downloaded the Modern Warfare 3 Dedicated Server under Tools on Steam. Then I installed the IW5M client with DLC's. Then in the root folder of the MW3 folder, create a folder with nothing in it named "dw". This allows free play without Steam. Only reason I downloaded Steam dedicated server was because the files are free of viruses you may find in pirated copies online.