Here's why:
* Linking verbs connect the subject of a sentence to a word that describes or renames the subject.
* In this case, "Susan" is the subject, and "confident" is the word that describes her.
* The sentence is saying that Susan's appearance *indicated* confidence, not that she performed an action of being confident.
Examples of linking verbs:
* be (is, am, are, was, were)
* become
* seem
* appear
* feel
* look
* sound
* taste
* smell