I was actually pretty happy when I noticed that. I think phones with really bright button look kind of cheap-ish and overdone (as most phones that simply install LED's under the keys have a very bright uncontrolled-lighting effect).
The amount of light with the Evo are enough to make them not look much more than the painted buttons as they appear with the light off, which I think is the desired effect. This way it simply looks like the painted button happen to also be visible in the dark, almost like they're not actually being "lit" but are just magically bright enough to see.