They are taking his refusal to do further inestigation as a complicit admission. However, it ignores all the drug tests he passed over years of competing (that is, every one he was ever required to take).
It ignores the fact that he has said he played 'within the rules as they were at the time' which leaves open the possibility that he did use things that were undetectable until they became detectable, though he has never admitted that.
It ignores the possibility that everyone else was doing the same, which is why the Cycling bodies didn't want to touch it with a barge pole - do they now have to retest everyone's decade old blood tests to see if they can find traces they couldn't at the time? Do they even have those blood samples in storage?
It expects him to try (without recourse to blood test results) to refute oral accusations from competitors and less successful team mates. 
It's tall poppy syndrome - no one thinks anyone could do what he did without cheating - but no one expected him to become competitive again after beating the cancer. If he did one 'impossible' thing, he could have done another.
I think it is a shame that USADA have over played this - they have diminished no one but themselves.