I also see "poor grammar" from non-native speakers. Including me: Duolingo was correct today when it told be that I shouldn't use "tu es" as a French subjunctive. No native speaker would say that. I just couldn't hear that what I typed was different from "tu aies" -- though I should have known from context.

You're absolutely right that much of what people tag as "poor grammar" is in fact a different dialect. It's perfectly natural to them and the people around them, following a set of rules that they know but may never have written down. They may evolve from mistakes that become persistent and thus valid in context.

Speaking in the wrong register or dialect is an error of its own -- not a grammatical one, but a semiotic one.