I think it`s not always so easy to find out if GMO food is harmful. F.e: long time effects. Another thing is: maybe GMO food isn`t really harmful, but isn`t healthy either. I don`t why you discuss this topic; if it`s ethical discourse then I would say, that it`s not for sure, to be for GMO food.
The claim "exports" the burden of proof and thus responsibility. If one engages in a risky (potentially harmful in any way) activity - in such delicate areas as food - one holds the responsibility of proving that the activity is not harmful, risks are minimal etc.