The funny thing is, people DO make maps from other games. Just look at that new Goldeneye Facility map. The problem lies when complete TARDS request other people do it for them. What kind of idiot for fall for that, besides more TARDS. Then they end up making the most awful replica and then rave about it, much to the other members disgust.
If you want to make a halo map, do it yourself with your internet buddies. Don't try and drag everyone else down to your level.