Improvising or writing onstage is a tightrope walk. And if you're talking about edgy/provocative topics, it can be even more dangerous. That's part of what makes it so thrilling to watch. Make it to the other side and it can be magical. Fall off and you wind up in a shark pit. (Is that a thing?)
Comics defend other comics for the same reason that cops defend other cops; We've been there before. When a civilian hears about an innocent suspect that gets gunned down, he's outraged: "How could this happen?" When a cop hears about it, he'll say something about the stress of being in the line of fire: "You'll never know what it's like to approach a suspect and not be sure if he's reaching for a wallet or a gun." Luckily for us, we don't actually kill when we "kill."
Personally, I take anything that is improvised on a comedy stage with a huge grain of salt. Because I've said plenty of in-the-moment things onstage I later regretted. As bad as Morgan or Michael Richards? Probably not. But there has definitely been plenty of stuff I'd have to apologize for if everything I said was reported by the media. Take chances onstage and you're bound to misstep occasionally. And once in a while, you may misstep badly. That's part of the deal.
(Note: I'd be more likely to condemn a written/planned joke than a riff. Malice aforethought, etc. But I'd still probably be way more lenient on that than an ordinary person. Also worth keeping in mind: I don't understand being offended.)
The conversation reminds me of Dave Chappelle's thoughts on Richards' meltdown at The Laugh Factory. While performing on the same stage, Chappelle explained how watching the whole thing made him realize he's "20% black and 80% comedian."
The black dude in me is like "Kramer, you motherfucker." I was hurt. And the comedian in me was just like "Whoa, nigger's having a bad set. Hang in there, Kramer. Don't let 'em break you, Kramer!"
Obviously Chappelle hates what Richards said. But he's been in the trenches too and can't help but identify with a guy who's losing it onstage while dealing with a non-receptive crowd. That's the empathy you get from walking in a comedian's shoes.
I'm not saying Morgan should have said what he said. I get why gay folks are especially pissed. I know the goal of those criticizing him is a noble one. But the road to neutered standup is paved with good intentions.
I believe the standup stage is a sacred space; It's one of the few places left where people are allowed to experiment, confront, and dance with ideas that society generally tiptoes around or avoids completely. That won't always go well. But if you try to take away the shitty part of that, you're likely to sacrifice the wonderful part of it too.