This is nothing new. It's the old "Sore Loser" syndrome that seems to infect certain segments of any gaming community's population.
Don't take it to heart, these are folks who, while admitting to themselves that the possibility of cheating exists, would rather delude themselves into thinking that they lost to somebody because that person was breaking the rules, rather than they themselves were lacking in some area of skill or ping.
These are the folks who give gaming a bad name, sadly. Perhaps they've been scarred for life by cheaters of some manner or another, but that's no excuse to accuse everyone of cheating just because they're tough to beat.
Almost everybody who's been beaten badly is tempted to whine or maybe feels let down. But the thing to do is to suck it up and try to learn from your mistakes and get better. Otherwise you're stuck in a downward spiral of complacency and mediocrity.
My advice? Keep on playing, and ignore those guys. You know you're not cheating, so just avoid them. If they insist on playing you, maybe they'll get beaten so bad they'll just leave and take their whines someplace else.
This didn't start with JK2, it's been around at least as long as games have been around (and sore losers). Take for example the old "I don't want to play anymore (intentionally knocks over board)" excuse when somebody loses at checkers. ; p