I don't think this is a frequent mistake among native speakers. In fact I remember hearing such wrong form, but not so frequent to be a pattern of the low educated people or some other social group.
In other verbs, also I don't recall to be hearing negative imperative taking the form of the affirmative one.
Maybe I heard "Nu zi!" instead of the correct form "Nu zice!"...
I guess that intuitively Romanians feel that negative is different in form than affirmative imperative (not a rule, but valid in 90% cases), so while speaking and searching the right word they usually say the correct form.
Also the fact that the negative imperative form is preceded by the negation "nu" offers the speaker a short delay to search in his mind the correct form, avoiding the error.
I mean: if the pattern of the negative imperative would be "fă nu așa!" or "zice nu asta!" I guess the errors in speech would be more frequent.