Spammers will post some text, then edit the post inserting images.
For this reasons Michael set edit back to 1 minute, which would allow him to review the post of new members and accept it, at same time knowing they can't spice it up.
What I do not understand is why people with 50+ posts have the same limit.
It's at a 5 minute time limit for now to make quick edits... Again due to issue of spammers. Regarding why it's that way across the board is because there's only one option exposed by vBulletin for time limit on editing posts. There might be plug-ins to make it dynamic but I haven't seen one and I don't have time to write one.