Just trolls being trolls.
I wish they had included a report for "Exploiting" like they did on MW3, as that is basically what they are doing, exploiting a feature of the game to ruin the experience for others.
Use the XBL reporting tools to avoid them and report them and leave the lobby.
See this from time to time in HC TDM, but not as bad.
Sad that people have "fun" by ruining the fun of others.