Basically it was set that way at the beginning to keep the polls at the top of the threads while people were interested in them. When people lose interest and stop voting they will go away. If the checkmark is removed, just voting will not change the position of the poll and it will get burried down the list somewhere. Unfortunately, it seems that most people don't go past the first page when looking at the forum, and this was an easy way to keep polls active and viewable for a few days.
I just checked my Admin. options and Morgan still has it checked. I don't think that we need a poll to find out if we need to change the poll settings,
but a few more comments about which way people would prefer would be helpful.