Native speakers don’t usually make major grammar mistakes. They may not follow prescriptive rules, but they’re generally understandable by other native speakers because grammar is so deeply embedded in their subconscious that they can’t help handling the language correctly. You do the same in your native language. Everyone does.
The problem with non-natives, and I include myself as a non-native speaker of a few languages, is that we don’t usually have the same instincts. It would be pretty arrogant to tell a native that they don’t know how to use their own language when we, almost by definition, cannot possibly understand it in the same way that they do.