They categorize you based upon what languager is spoken in your country, it's that simple. I would think the meaning of "native" speaker would be obvious, your a native of a country where "English" is the spoken language. In America most everyone speaks English, hence, if your from there your a "native" English speaker.

If however your from Spain, your not, get it?

It doesn't matter if you have no accent, odds are you'll never speak it as well as an accomplioshed native speaker. Maybe you speak better than your Homey in the hood, who has little education, but that's not what the majority of schools here look for. In truth though, they just play to parents and adult learners preconceptions in regards to who teaches the best English.

It might also be that you speak it well enough to teach it, native or not. However, the preconceptions of parents and most students say that you do not, so they don't want you.

In the end it is all about perception being reality, and in reality, you are perceived as an inferior product, regardless of your actual ability. Hope this post helps.