If you wanted to play Call of Duty, you'd play Call of Duty and not Halo: Reach. Unfortunately, it's not always up to you. Many times when you think you're playing Halo: Reach, the yahoos in the lobby pick the SWAT variant, which takes away shields and radar, and gives everyone a gun that rewards headshotters. Viola! Now you're playing Call of Duty. You can't even get away from that game when you're not in that game.
Fortunately, Bungie will fix this by segregating all the Call of Duty players into their own SWAT playlist. They've posted their plans for playlist changes coming with a mid-October update, which will also let you play the campaign missions with the kinds of random yahoos who vote for the SWAT variant.
SWAT Playlist added (SWAT removed from Big Team Battle and Team Slayer)You can also read about some new variants and minor tweaks coming in mid-October.
Campaign Matchmaking (live on 10/15 - You will be required to have earned the Grade "Corporal" to enter this Playlist)
Team Slayer rebalanced to highlight default Slayer game variants and include new offerings
Classic Slayer removed
Removed "Pro" game variants from the Arena Playlist
Removed Boardwalk from Arena Team Doubles Playlist
Added "Return to Battlefield" volume to Space on Zealot in the Arena Playlist