This wasn't the case. They would have pulled the plug on his immediately if they thought he went that far over the line. A "breaking news item" could have run, a rerun could have been run, they could have cut to a commercial, etc.
Consider if an on air personality suddenly started voicing holocaust denial talking points. There is no reason to leave someone on the air (even till the end of their own show) in some circumstances.
It's not government's job to determine this, it is the job of standards and practices. This was a decision that was held until the heads could be brought into it (and possibly an email campaign started).
It is a double standard though because other "offensive" hosts have remained on the air until their ratings fell and viewers notified advertisers of a boycott.
I don't think that NBC needed to wait for that to happen (and I don't know that it would have happened) but certainly there is more patience shown with leftist hosts and little if any lastig outrage.
A Florida liberal talk show host announed "news" that Katherine Harris died in a plane crash (and he followed this with "Yes!!!"). The story didn't even go national.
Katherine Harris alive despite Miami radio host's announcement (hate radio)
As to your Dixie Chicks comment, McCain had them come to Congress to speak out against their blacklist today. The Republicans do not work towards improving the double standard.