You've kinda lost track of object reality there: "Only logged in users who are deliberately un-hiding this content" is hardly "as valid as any other content".
Slashdot's attitude is that things which are "removed" (voted to zero) ought to be auditable. This means everyone knows what the rules REALLY are (as opposed to what people SAY they are), and it also prevents the system from being abused (since you can trivially link to examples of that abuse)
I really don't think an auditable record of moderation decisions is that bad, and you have to understand that this is NOT "content that is presented to regular users." I don't remember my login anymore, so I can't even figure out how to see the comment you linked - I'm sure if I logged in and reset myself to view negative scores I COULD see it, since I've used the site before, but it is distinctly non-trivial to do so.
