Spelling, grammar and sentence structure are not taught as much as it was years ago. My grammar is usually bad from laziness as I usually type however I am thinking it. However, My spelling is usually good. I think another reason that people who learn english who are not American write English better, is that they learn proper English. Generally, in the US we speak dialects and that's how we generally write. Very few places in the US speak "proper" English. This does not excuse bad spelling though.