One thing you could say that might help is if you were clearer about when you consider it evil to ignore the volition of an intelligence, since it's clear from your writing that sometimes you don't.
For example, "don't be evil" clearly isn't enough of an argument to convince you to build an AI that fulfills Babykiller or Pebblesorter or SHFP volition, for example, should we encounter any... although at least some of those would indisputably be intelligences.
Given that, it might reassure people to explicitly clarify why "don't be evil" is enough of an argument to convince you to build an AI that fulfills the volition of all humans, rather than (let's say) the most easily-jointly-satisfied 98% of humanity, or some other threshold for inclusion.
If this has already been explained somewhere, a pointer would be handy. I have not read the whole site, but thus far everything I've seen to this effect seems to boil down to assuming that there exists a single volition V such that each individual human would prefer V upon reflection to every other possible option, or at least a volition that approximates that state well enough that we can ignore the dis-satisfied minority.
If that assumption is true, the answer to the question you quote is "Because they'd prefer the results of doing so," and evil doesn't enter into it.
If that assumption is false, I'm not sure how "don't be evil" helps.
Taken from some old comments of mine that never did get a satisfactory answer.
1) One of the justifications for CEV was that extrapolating from an American in the 21st century and from Archimedes of Syracuse should give similar results. This seems to assume that change in human values over time is mostly "progress" rather than drift. Do we have any evidence for that, except saying that our modern values are "good" according to themselves, so whatever historical process led to them must have been "progress"?
2) How can anyone sincerely want to build an AI that fulfills anything except their own current, personal volition? If Eliezer wants the the AI to look at humanity and infer its best wishes for the future, why can't he task it with looking at himself and inferring his best idea to fulfill humanity's wishes? Why must this particular thing be spelled out in a document like CEV and not left to the mysterious magic of "intelligence", and what other such things are there?