Hi-
This option is frequently limited or even disabled on forums for a couple reasons, which essentially boil down to a few bad eggs ruining it for everyone.
Sometimes discussions escalate to a point that greatly upsets a member of a site. If the edit option is left open, this member can go through their old posts in a 'rage quit' and change everything to "deleted" or often more vulgar. This causes a lot of confusion as it ruins the flow of conversation in threads. Sometimes these members have posted great Tutorials as well, and editing those out would really hurt the community.
Less extreme but equally disruptive are the members that post something, get a few replies, and then change their original message to something different, again ruining the flow of conversation.
Or if a member asks a question and gets an answer then changes their question post to just "thanks - resolved", another member with the same question would not be able to easily find the answer and would need to ask again, which hurts the 'archival' aspect of a forum.
Dayle