Yeah, I really hate it, too. I think in all of my time playing (only MW1, W@W and MW2) I have only been in a lobby with good sports a handful of times. One was last week and we were getting slaughtered. Played 2 games of S&D with them, lost 4-0, then lost 4-1. During the killcams they wouldn't gloat, they didn't do a LAME spin diffuse and they said "good game". It was refreshing. Don't really experience that a lot.
For a company that spent so much time and energy exhorting the virtues of a free online service, to charge for marginally useful access and other nebulous online things, this is just another slap in the face of PS3 owners who were previously told that rumble is last gen, backwards compatibility is great and the versatility of their system in allowing other operating systems to be used on their hardware was a great feature.
- Lono, Sarcastic Gamer
If you ask me why, all games have some groups of idiots online, but a lot of them are concentrated in COD since it's easier and more popular.
As for why they would get worked up over it in the first place, I don't know. A bunch of people have made this into their career or something. If you happen to excel at it, they may see you as a threat to their "status." In order to secure their status, they have to challenge you in some way or another, usually with some verbal abuse. The anonymity of the internet gives much courage.