These are the same people who criticized the Bulls for keeping Derrick Rose on the floor late in the game during which he tore his ACL. These are also the types of people who probably would have ripped the Bulls if they had taken rose out and the 76ers had managed a Clippers-esque comeback.
These are the outcome-based second-guessers, and they are infuriating.
Folks, this is not the same thing as lamenting the fact that a knife-juggler cut himself. By all means, criticize away in that case -- because that is an inherently dangerous act. Rivera? He had been shagging flies for the better part of his Hall of Fame career. He loved to do it. Maybe in some strange way, it helped him stay loose and connected -- and made him a better pitcher. Is it more dangerous than, say, sitting quietly inside of the clubhouse? Marginally, yes. But we can't live our lives inside of plastic bubbles. Accidents are going to happen.
So yes, feel bad for Rivera's poor luck. Lament the fact that one of baseball's all-time greats -- by all accounts a very good human being, too, even if he is a Yankee -- might never pitch again. But please don't make the illogical leap and say he never should have been shagging flies in the first place.