"as" is ambiguous - it can be interpreted as "because" or "the way that".
Itheann tú mar ithimid does NOT mean "You eat the way that we eat", it means "You eat because we eat".
Translation mar as "as" in this exercise would be at best misleading, and, as (!!) there is a much clearer and unambiguous translation available, only "because" should be accepted.
Hmm. It might be the English rather than the Irish. I remember hearing a story about Walter Cronkite objecting to a cigarette commercial slogan, "Winston tastes good, like a cigarette should," on the grounds that Cronkite thought it should be "Winston tastes good, as a cigarette should." I suppose Cronkite's point was that "like" is not a conjunction. I will leave that debate to the grammarians. I am suggesting only that perhaps duolingo accepts "you eat as we eat" but rejects "you eat like we eat" on similar grounds.
Yes. This is problematic. "You eat 'because' we eat" is saying something very different from "You eat 'like' we eat". Using the word "as" leaves the meaning ambiguous - it could logically go either way.
I believe that "You eat 'because we eat" is better said as, "Itheann tú toisc go ithimid."