Because that's a conceptually straightforward assumption that we can safely make in a philosophical argument.

But this is not a philosophical argument.

To recap:

• I suggested that an AI which is a precursor to the FAI should come to understand human values by interacting (over an extended 'training' period) with actual humans - asking them questions about their values and perhaps performing some experiments as in a psych or game theory laboratory.
• You responded by linking to this, which as I read it suggests that the most accurate and efficient way to extract the values of a human test subject would be by carrying out a non-destructive brain scan. Quoting the posting:

So when we try to make an AI whose physical consequence is the implementation of what is right, we make that AI's causal chain start with the state of human brains - perhaps non-destructively scanned on the neural level by nanotechnology, or perhaps merely inferred with superhuman precision from external behavior - but not passed through the noisy, blurry, destructive filter of human beings trying to guess their own morals.

• I asked how we could possibly come to know by testing that the scanning and brain modeling was working properly. I could have asked instead how we could test the hypothesis that the inference from behavior was working properly.

These are questions about engineering and neuroscience, not questions of philosophy. The question of what is right/wrong is a philosophical question. The question of what do humans believe about right and wrong is a psychology question. The question of how those beliefs are represented in the brain is a neuroscience question. The question of how an AI can come to learn these things is GOFAI. The question of how we will know we have done it right is a QC question. Software test. That was the subject of my comment. It had nothing at all to do with philosophy.

You make AI correct on strong theoretical grounds, and only test things to check that theoretical assumptions hold in ways where you expect it to be possible to check things, not in every situation.

Ok, in this context, I interpret this to mean that we will not program in the neuroscience information that it will use to interpret the brain scans. Instead we will simply program the AI to be a good scientist. A provably good scientist. Provable because it is a simple program and we understand epistemology well enough to write a correct behavioral specification of a scientist and then verify that the program meets the specification. So we can let the AI design the brain scanner and perform the human behavioral experiments to calibrate its brain models. We only need to spot-check the science it generates, because we already know that it is a good scientist.

Hmmm. That is actually a pretty good argument, if that is what you are suggesting. I'll have to give that one some thought.