These "voters" (I'm hoping they're not actually voters) are idiots. But MSNBC doesn't help matters by saying "McCain forced to defend." They make it sound like McCain really wanted to let it slide but some law said he had to defend Obama, not that he did it on his own.
YouTube - McCain forced to defend Obama vs. hateful ignorant mob
As for that clip where he "did nothing" I'm going to guess that McCain didn't actually hear the guy scream "terrorist" I hardly heard it myself. But I suppose it's more exciting to suggest he encouraged the vitriol.