I have staff experience. I like the look of your forum but there does seem a lack of posters. So I wont commit myself just yet.It is just a standard setting on discussion forums. If you make a typo or something like that chances are you will spot it within the 15 mins limit.
If someone needs to delete or edit their post(s) after that, the chances are they are disgruntled for whatever reason which can ruin the threads for other users if some of the entries are missing or defaced.
You can still contact staff to have your post edited if their is a genuine reason and I have the option of creating a new (trusted) user group for users with a high amount of posts that have such restrictions removed.
I will also, if the forum gets busy enough be looking for moderators who not only can edit and delete their own posts but posts of others too. eg spammers.