That is not the reason they went from 2.1GHz to 2.0GHz
The nVidia graphics in the 2009 Macbooks use more power than the Intel graphics in the 2008 Macbooks. To compensate and ensure that overall power consumption was about the same, Apple switched from a standard voltage CPU to a low voltage CPU, and the closest available speed for a suitable CPU was 2.0MHz. (The 2.0GHz CPU also has a faster front side bus, so it is probably a slightly higher performance CPU regardless of the slightly slower clock speed anyway).
It may be that Apple also wanted the white Macbook to have a slower spec for marketing reasons, but this reason isn't necessary. It makes perfect sense for purely engineering reasons.