It's about maximizing profits. Making just "a little bit more" money. Beating profit targets, etc....
A Program Director would be run out of the business if he accepted that "ah heck, I'll make 90% of my station revenue from Rush" - because if that was all that mattered they'd sell you the time for free after 7:00 PM, and they don't (I've been with a couple clients/organizations who have bought that time).
Don't mean to drag this on, (besides, I'm celebrating Zarqawi's descent into Hell, as I'm sure you are) but, in fact, stations virtually do give the time after 7 away "free". They do this with packages that include spots in the evening to "flesh out" the schedule, and keep the "average" of the spot cost down. It's called ROS, or run of schedule, which means, generally, that you buy the entire day, 5 AM to 12 midnite, and the spot rate is less than if you bought a specific day part such as AM or PM Drive, or in KFBK's case, the Rush Limbaugh show.
Rarely have I ever seen someone specifically ask to buy time after 7, although I'm sure it's been done.
You're right, however, that stations want to maximize profit, and in my first post on this subject, I said the reason for Williams release was money. I stand by that, and in fact, a month ago, when the Bee first reported that Williams was gone, I think money was the cited factor. I don't know what difference in salary there is between Williams and Maiman, but I suspect that Williams was getting to be high maintenance in a lot of other ways, and I also suspect he may have come with some baggage from the manager who had originally hired him (now departed from KFBK)
Bottom line, income revenues won't be affected by this move, and there may be some slight savings in personnel.