ネタ can mean alot of things. for comedians neta=material. In sushi-terminology neta=the part of the sushi that isnt rice (the meat or main ingredient). In both cases i think its used pretty interchangeably with 材料. Some people say its derived rom タネ or 種 which means seed. So rather than implying transmission or dispersion i think it has more to do with the ネタ being the base material or the root from which something sprouts (as in a gag or news item or business model).