A lot of Flyertalkers will qualify in an inexpensive way for the promotion, and may or may not remain loyal to KLM after the promotion finishes.

At the same time, KLM may pick up a lot of ex-Qualiflyer passengers who fly paid business class. These customers may give KLM twenty paid segments in international C-class, and continue flying well after the promotion has ended. KLM are assuming that top-tier flyers are high-revenue flyers -- and in most European programs, a lot of top-tier flyers fly paid C-class or paid F-class. This is less frequently the case in US programs.

I suspect that for every FTer trying to qualify on cheap tickets, there are several non-FTers who qualify through their normal business travel routine. When you look at the incremental cost of providing an award seat, even the millions of miles that KLM are handing out are justified if they attract enough of these high revenue business flyers.