This is quite usual for posts across most discussion forums. The editing window is there to allow the poster to catch spelling mistakes and tidy up grammar etc. If there is a major mistake that needs pointing out at a later date then just put your hand up and post a correction. Note that disallowing later edits also helps to avoid following posts to appear out of context which can be even more confusing.
Lesson to be learned is to be absolutely sure that what you are posting is what you actually want to post in the first place.
gomezz is absolutely right. If there's anything really egregious, you can also ask a Mod to make an edit, but it's usually not absolutely necessary -- a followup post with the correction should suffice.
When I was a mod at other forums you would get modified posts where the poster would try to add a spam link after the initial post too . I think this way keeps the timeline easier to track I would think as well.