They are prescriptive by extension, e.g. when describing usage from prescriptive speakers. Language is not inherently descriptive or prescriptive, it is a fals

They are prescriptive by extension, e.g. when describing usage from prescriptive speakers. Language is not inherently descriptive or prescriptive, it is a false dichotomy, because language is acquired by copying, but it is is modified to suit needs, so it is used prescriptive. If language was purely descriptive, where did it first copy from? In a sense, I assume that language faculty is inherently prescribed in a natural way. And in another sense, the words show de-scription is copying (this doesn't work well in english, but in french e.g. it would be quite literall "ecrire de ..."), and pre-scription is what was written before it was copied. It's two sides of the same medal. If they describe, somebody else will interpret that as prescription, whether that was intended or not.