Might that be chalked up to "cultural sensitivity" toward seeing a child commit such an act of violence? Zombies, sure.. But an actual human life?
I know it's a stretch, but with the way networks think, these days, I wouldn't be surprised if that was the reason.
I'm pretty sure the reason was that Rick had to kill Shane to not look like an even bigger idiot than he already is. In the comics Carl saving Rick worked because Rick had no reason not to trust Shane, in the series after everything that happened it would have been ridiculous if Rick had walked into Shane's idiotic trap unprepared. No one in the audience could ever accept Rick as a leader if he had done that.