Because MW2 is a dumb game online.
Oddly enough it's hard to acctually camp on CoD games in general due to the map design that is more for close quarter combat, but bottom line is quickscoping is the most retarded tryhard nonsense that I have ever seen in a competetive game (if you could consider MW2 a competetive game).
Yes, you can have quickscoping. Quickscoping is fine! But it bugs me to no end when people act like 13 year old scrubs just because you use the sniper as it was intended.
It reminds me of those mashing tards on street fighter 4 who get really salty after losing a match because I don't want to rush down someone and think that blocking is overrated.
Okay that's only happened to me once, I can imagine anyone who sends a message to me online on street fighter acting like that is probably a CoD player.