Notice that there is a place for both description and prescription in language study. For example, when adults learn a foreign language, they typically want someone to tell them how to speak, in other words to prescribe a particular set of rules to follow, and expect a teacher or book to set forth those rules. But how do teachers know what rules to prescribe? At some point in time, someone had to describe the language and infer those rules. Prescription, in other words, can only occur after the language has been described, and good prescription depends on adequate description. We obviously don't want to be teaching people the wrong things about language.
Linguistics takes a descriptive approach to language: it tries to explain things as they actually are, not as we wish them to be. When we study language descriptively, we try to find the unconscious rules that people follow when they say things like sentence (1). The schoolbook approach to language is typically prescriptive. It tries to tell you how you should speak and write.