Tough crowd? Or one completely within its rights?
First, let's move past the are-the-fans-allowed-to-boo discussion. It's a resounding yes.
The fans pay good (or exorbitant, depending on your viewpoint) money to attend these games. When the performers don't provide a good show, feel free let them have it.
"But they're humans, and no humans should be treated that way. How would you feel if people booed you at your job?"
Your boss doesn't pull you into his office and ream you out if you aren't pulling your weight? What's the difference? The fans pay these guys' salaries and are responsible for their jobs. In that way, they are the bosses of these players. Those salaries are pretty high, too.
Now, as for booing the current quarterback of the Chargers, go ahead. He was not living up to his end of the deal.
Rivers was once an elite passer. He was in the discussion whenever you spoke about the best five quarterbacks in the league.
Last year, Rivers struggled terribly with interceptions, with 20, and fumbled seven times.
That trend carried over to 2012. Thus far this season, Rivers has thrown 10 interceptions and put the ball on the carpet five times.
But it was the last interception that brought out the frustration of the fans. On third down from the 1-yard line, the Chargers elected to utilize a play-action pass. As Rivers continued to roll out with defenders chasing him, he threw it toward a covered Dante Rosario in the back of the end zone.
The resulting turnover ensured that San Diego wouldn't add to their lead before halftime, and the fans let them have it.
Yes, Rivers was pretty accurate during the first half, but this was about something different.
And while the Chargers eventually got things going (or perhaps, more accurately, the Kansas City Chiefs remembered that they're the Chiefs), that doesn't excuse such unintelligent plays. These aren't even questions of talent, but just poor execution and dumb decisions.
So go ahead and boo away when your team refuses to perform to its capabilities.
Your boss would.