I would assume it's a "feature" to keep people from changing questions after people answered the poll.
IE:
Original question - Do you like pie?
Edited question after 100 people answer - Do you like to drop the soap?
Or how about if the question changes all the votes get invalidated....
As far as anyone can determine, vBulletin has no provisions for members to edit polls. Sorry 'bout that. If you make a poll and need it edited, you can post in Mod Discussions or PFI and a moderator will have to do it for you. That's the best we can do.