Many stations not only pay for Limbaugh, but pay so much that his show is a loss leader---meaning he is used as a draw to the station, but because of his rates, they really don't make a lot on his show.
And on top of that, premiere has those national avails.
As far as this two-week suspension of national spots:
It seems that Clear Channel is just taking the heat off of those sponsors who may want to stop temporarily. This way, nobody stands out and risks a backlash, which only further complicates the sponsor-affiliate situation.
Smart move, actually.