The user starts with 1 in order to have the privilege to post the questions or answers. Theoretically zero would mean that you don't have reputation at all then what would be the point of creating the account if you can't do anything. So it doesn't go lower that 1, otherwise one down-vote could take away your ability to post anything.
See also:
- Help Center > Privileges > Create Posts
select top 100 u.Id as [User Link] from Users u where u.Reputation <= 0at Data