Forum policy has been changing over the years. It is in direct relation with the level of maturity, tolerance and behaviour shown by members.
Initially edit was made available. At some point of time it started being abused. People would post inflammatory comments over corolla/civic debate or some racist/ethnic/political issue. When a full-fledged war would start, they would go back and erase/alter their comments, giving moderators no chance to know whom to hold responsible.It is not feasible for the forum to block or warn so many members who just got worked up on an incendiary comment, and the root of the problem needed to be identified so that the true miscreant could be thwarted from future infractions.
In this situation edit option was taken away from members and only moderators had it. After some time when edit option was reopened there was an important working behind it. The entire history of the post was available to the moderators as well as the users. If you edited the post 20 times, there would be 21 versions of that post in PW record. For a few weeks, users could also see all the versions of their post. Compare tool was also available (similar to Wikipedia edit tool). Compare option was taken away from users due to whatever reason, but I think so PW still manages the history of every post so even in case somebody edit their initial post, the latest version will show online but in the database, the old versions would be present for potential Report Abuse complaints.
When you remove post, you remove (I presume) not only all versions of that post but also the fact whether there was a post here in the first place cannot be ascertained. Therefore delete post is only available to moderators.
Meanwhile at some point of time some moderators also started over aggressive moderation, to which there were many complaints. Old contributors like Xulfiqar and others even got disheartened and made threads that they want to leave. Moderator training programs were conducted and threads like What do I do if I have a complaint of moderator? were published by the company. Another step taken was that moderators had to declare that 1) the post has been moderated. If the user posted something and a few days afterwards he/she sees some text added/remove/altered, there must be a moderator note there. 2) Moderators should write their name, so it could be tracked down who did the moderation. This policy worked for a while and did wonders to taming the behaviour of moderators, whose aggressive moderation was chasing the old contributors going away. After a while the policy was reversed because there was no reason to declare the moderator since it started more fighting than the good purpose it served.
In the threads of that era, you can witness a lot of toofast edit, etc. etc. in the posts.Now I hope you understand how the policy is fluid, how it changes and why it changes.If you have made a duplicate post by mistake, a good way is to erase it and type "Self edit: duplicate" in it. Many people do thus. Then when any moderator sees it, he/she deletes the post.