Firstly, I do not think Ethiopian consumption is high enough to make up for the potential lost tariffs. Ethiopia is less populous, poorer and less developed than Rhomania.
Secondly, as far as I know, Ethiopia currently has two major sources of revenue: the coffee trade and the Red Sea trade, and the second is by far the more valuable. When Ethiopia has a fleet to maintain in the Indian Ocean, they need as much income as they can get.
Thirdly, did I not say that this idea would mostly be used as a bargaining chip? The Ethiopians likely won't want to sour their relations with their largest trade partner and most powerful neighbour, so this whole threat is mostly meant to be the forewarning. It isn't like either the Romans would want to go ahead with it nor the Ethiopians find out if the Romans would go ahead with it.
Lastly, given that Roman merchants in the east have suffered heavy losses in the east, provided that the government gives them (the ones who weren't committing tax fraud at least) the tax break they want, they're not in any good position to argue.
