Bungie (or any game developer for that matter) has no control over the development of aimbots and other mods for their game. The problem itself is with the the Xbox 360 console, and how people have discovered a way to bypass Microsoft's security measures that stop the Xbox 360 from running unsigned code (Microsoft patched this, but many people had JTAGs, or modded Xbox's that can run unsigned code, before the update). Because of this, people who own JTAGs can experiment and manipulate the code for Halo: Reach however they like.
Bungie can ban their account from matchmaking, but it's very hard for them to prove anything unless the guilty party uses the mod/aimbot many times in a row. If they got permanently banned from Halo: Reach matchmaking, the person with the aimbot would most likely just make a new account (they're probably using a one month to begin with). People with JTAGs can't be console banned because they can unban themselves by just using a new KV.
