There are clauses in the End User License Agreements of most games that are supposed to restrict the amount of modification a buyer can perform on the game software itself. The restrictions are weak enough that there's a lively trade in trainers and third-party mods, and it's only a small stretch from there to no-CD cracks, and from there to full-blown duplication of the rules. The issue isn't so much the regulations in place in this instance as it is the ability to enforce EULAs and other limitations on modification/distribution of game software. Whether or not the mod in question was 'legal' is a very grey area.
