Quite simple, to keep the price's of the xxD line artificially higher, it's
nothing new companies do it all the time. It's cheaper to use the same hardware and just disable stuff in software than to make a whole other line of sensors.
You're still missing the point. The XTi was the current Rebel when the 30D was released. The 30D didn't have native ISO 3200, and the 40D didn't have native ISO 3200. Both were software pushed. So two generations of cameras that COULD have had native ISO 3200 without cutting into sales of anything didn't for what reason?