As dglienna eludes, there are times when people aren't allowed to allocate points either positive or negative, but can still "rate" a post. If a person has not been on the forum long, if they don't have enough posts, or if they have a reputation below a certain level (such as a negative reputation), then they are not allowed to allocate points. These people can, however, still select to rate a post as positive or negative to show their opinion. Their rating simply doesn't impact your reputation.

Clear as mud?

Brad!