I don't know that there's really an answer to this in Linguistics. The issue I think you're dealing with is whether correcting people is right or if it's wrong (because it does something like destroying linguistic diversity or elevates one form above another). After all, they do speak the language too right? What's to say that my corrections are more true to how I would say it but not truly necessary.
My personal opinion is that there are often registers of a language that have different guidelines compared to other registers. Correcting which register is used is okay in my book but saying that some other register is not English or something like this is not. For example, if you're reviewing an academic paper and you use informal language, I don't think it would be out of line to suggest ways to shift the writer's register to something more formal. However, is it fair to call someone out for using contractions on a web forum? I would say not but really I think this is a matter for individuals to make up for themselves.