I'm sure the vast majority of "loose" spellers are indeed native English speakers. I have no idea what causes it, either. "Win" and "lose" are some of the basic vocabulary words a kid learns. It's used in sports and competition all the time. There's just no excuse for it. There's not too many words ending in "-ooze," but no one spells "booze" as "boose."

Mixing up words like "their" and "there" are a little more understandable, since they sound alike and people might sound words in their head as they type.

And geez, how many people spell the Eminem song as "Loose Yourself"? It makes me wonder if people saw the title of Nelly Furtado's album and wondered why she called it "lose."

And yeah, "loose" can technically be used as a verb in its own right, but it seems more or less an obsolete usage.