'Seen as' or 'seeing as'

Solution 1:

Seeing as is the generally accepted form; I don't recall ever hearing anyone say seen as (though it's possible I just didn't notice).

Grammatically speaking, seeing as is a conjunction; it fills the same function here as because. The Oxford English Dictionary defines it this way:

seeing that, hence ellipt. seeing: considering the fact that; inasmuch as; since, because. Also (colloq.) with as (how).

Its history goes at least as far back as 1504:

1504 W. Barons in Paston Lett. & Papers (2004) II. 501, I wol‥exhorte you to take it as‥paciently as ye can, seeyng that we al be mortal and borne to dey.