Holy cow, I wrote a book!
Commenter John Topley wondered
why clearing document history also clears Run history.
Simple: Because people wanted it to.
Originally, the button merely erased your recent document history,
with the increasing paranoia over what records the computer kept
on your past usage,
people began to expect more
and even filed beta bugs saying,
"I cleared my document history, but I went to Location X
and the names of documents I used recently was still visible."
I guess these people were afraid of being
raided by the
or more likely (but nobody will admit to it) by their spouse.