In an effort to curtail cheating, Valve has updated their rules on gift giving (thanks MCV). VAC and Game Ban enabled games must now be installed or gifted immediately. They can no longer be stockpiled in inventories for trade. This page has a complete explanation, and here's part of the reasoning:

When a user is caught cheating, their Steam account is banned from playing multi-player on secured servers. This creates an incentive for people who cheat to stockpile copies of games on one account and to gift them to low value accounts where they can continue cheating. In response to this problem, Steam no longer allows users to purchase copies of VAC or Game Ban enabled games to their inventory. Additionally, if an account has directly gifted a game to other players who have been VAC or permanently Game Banned, then that account will lose the ability to gift that game.