It's a business. The five biggest marathons in the US are New York, Chicago, Honolulu, LA, and MCM. Except for LA, those are all heavily advertised by Team in Training. So why wouldn't someone running a marathon reserve spots for them? You get TONS of free publicity.
So what if the average finishing time has ballooned by an hour since 1970 (3:32 to 4:32 for men in 2005)? So what if 30% of your field either doesn't bother to show up or doesn't finish?
Eh, my elitism is showing. A couple of good reads:
http://www.marathonguide.com/features/Articles/2005RecapOverview.cfm
http://www.slate.com/id/2149867/