I say those things because I want to bring awareness to the individual that they have matters they need to improve.
I don't say them because I feel like correcting every mistake I see, or because it "bugs me" to see things incorrectly spelled or punctuated, or the most irksome reason I find, that I'd simply do it to annoy the target, which is the reason when someone corrects a request to stop correcting grammar mistakes.
When I say something like that, it isn't because I recognize that someone wants to save a second by missing a letter or capitalization. It's because I truly believe that they need to learn what they are saying.
Perhaps there is no difference between the classical idea of a 'grammar nazi' and my own practices. But that just makes me a hypocrite, no crime I haven't been guilty of before.