I don't know but if I had to guess it's because some of these links could be abusive/harmful or spam. I don't think it makes much difference practically in the cases that they aren't since then it's not hard to recover the link with the info left, if you wanted to.
It doesn't address the specific thing but you can see the broader logic here:
Very likely flagged dead by users, as most things that show up as [flagged][dead]
Is that normal?
It is for dead submissions. Has been the case for ages.