Prescriptive Versus Descriptive Quotes

Deleuze and Guattari have been totally misunderstood because the following has been wrenched from context: "Forming grammatically correct sentences is for the normal individual the prerequisite for any submission to social laws. No one is supposed to be ignorant of grammaticality; those who are belong in special institutions. The unity of language is fundamentally political." (112)They are NOT advocating for this sort of prescriptive approach to language; rather, they are describing the social system around language--how language is a political tool. Why persist in quoting them as though they are promoting some sort of linguistic purity?