Bacause the people posting it have friends who will just post it again.
And some people will not read the correct info, but instead will take the disproved data and run with it.
[Just like CBS with the Killian memos and the mysterious fake CNN memo that the media recently ran with, same behavior.]
Exactly. Dead on. Bad information will be used and propagated regardless if it was disproved later in the thread. That is why bogus threads and posts are deleted.