Here's why:
* Direct objects receive the action of a verb. The verb "tastes" is a linking verb, not an action verb. Linking verbs connect the subject (salsa) to a description of it (spicy).
* The word "spicy" is a predicate adjective, modifying the subject "salsa". It describes what kind of salsa it is.
Let me know if you have other sentences you'd like to analyze!