The age-old excuse is that "people will abuse that feature."
This is why we also aren't allowed to go back and edit our posts after a certain amount of time (usually an hour).
To be fair, I can see both sides of the fence here: on the one hand, there have been occasions where people will go back and edit, and/or delete their old posts, and it makes specific replies to that post suddenly make less sense, and seem confusing - I can see the reasoning behind that. However, on the other hand, it's not a very common occurence, so I don't enabling those restrictions really justify the means for them . . . it's like a forum version of us all having to be molested to get on a plane, just because of a few bad seeds . . . or, all of us having to empty our pockets, and take off our shoes, to enter a public event, just because of a few bad seeds . . . or, all of us having to have an I.D. check to buy cold medicine, just because of a few bad seeds.
And, in Ed's case . . . I mean, yeaaah . . . deleting the entire thread, instead of going through each individual page (which is sometimes over 100) to look for one specific member's posts is a lot easier and less time-consuming, even if it isn't fair to the rest of us.