Because they can.
Otherwise, I agree with you that it's dumb as the bottom-end chips are always the best deal since you can OC them a greater percentage versus stock than you can a higher-end chip. In fact, the top OC clock speed gotten by *all* chips of a specific line is pretty similar- about 3.0-3.2 GHz for AMD chips and 3.2-3.6 GHz for Core 2s.
I also don't overclock, so I just disregard the overclocked CPUs in the metric, which is what I think anybody who doesn't OC should do too. If they do OC, if they want the highest possible OC, then go with the highest stock-clocked chip. If they want the greatest bang-for-buck in overclocking, always get the cheapest CPU in the line. That's darn near written in stone as far as I have seen.