A lot of valid email addresses are obvious typos. steve@gmail is perfectly valid but useless in most web forms, for instance. A lot of websites drop technical compliance for the convenience of people who don’t know how email works.
Technical compliance can also become rather annoying when you start doing things like escaping characters in quoted strings or include spaces. Practically nobody is using any of that stuff in the real life, so you rarely ever need full compliance.
I don’t know why single character email addresses would fail that test, though.