Quit flaming over stupid crap or you'll be banned.
I don't claim to know how H3's engine works or what ODST runs off of, but I think it's safe to assume that ODST's engine is at least a modified version of H3's; therefore, all these "omg wtf bungie explain" replies are unnecessary, in my opinion. Reskinning is indeed possible, as we can see with the 7-Wood, but adding entirely new weapons to the game brings with it another slew of complications, such as compatibility with older maps or people who don't have the content.
Do these weapons come only with the ODST version of the maps? What about if they're made available for download from XBL? What if someone with an ODST version of Citadel puts an Automag on that map, then someone who doesn't have ODST downloads it? Or maybe someone with ODST set the silenced SMG as a starting weapon and then tried to play on Valhalla (a potentially different engine). Too many complications, especially considering we already have a BR and the silenced SMG is just unnecessary in MP.
These weapons probably would never show up in Matchmaking, which means thousands of people would start complaining about that, putting us back at square 1.
These are purely hypothetical situations, not official examples.
[Edited on 06.12.2009 3:26 AM PDT]