My understanding (and while I've talked to people who are in the working group, I'm not in the working group or following the discussions closely) is that this was already (prior to the spec being modified) the common understanding of the people within the working group. It just hadn't previously been written in the spec, perhaps because it was thought to be obvious.
(And, given the context of what the group is trying to do -- build a do-not-track mechanism that advertisers are willing to honor -- I agree that it's obvious.)
(And, given the context of what the group is trying to do -- build a do-not-track mechanism that advertisers are willing to honor -- I agree that it's obvious.)