The posts aren't removed, they're still there. Most people, when they subscribe to a new newsgroup, ask the newreader to download posts for the last week, or the last month.

But if you're posting from a web based forum, you have the whole lot available to you (after a fashion) without asking them to be downloaded. If you're really enthusiastic (or a spammer) you search through for all the topics of interest to you, and then may feel moved to reply. If you don't look at the date of the last posting to the thread, you end up digging up a thread that is long dead.