I'm fairly certain that they wouldn't sue gamers for modding their game, but editing files to get access to DLC they're selling is a different matter entirely. It may not seem like it to us gamers, but to them and their lawyers it very much will. While it's debatable from now to the end of time if putting Paid For DLC on the game disk is a jerk move or not, the companies who make the game and are selling the DLC won't look at any community with high regards for giving away stuff they're selling. And while I doubt they'd go as far as to sue individual players, it's far far easier for their lawyers to send a Cease and Desist notice to the head guys here at the Nexus and/or use a legal boot to the face to get the site shut down (or if we're lucky, just the XCom section).