You are saying we could stop robbing by leaving fortified. That is incorrect. Fortified does not give you -50% tpa on incoming ops. That would mean robbing someone into fortified (from normal stance) would be twice as easy.
It is -75% damage per op with a zero (or likely zero) mod to tpa when opping through double fort. Fortified, presumably, is -50% damage per op with a zero mod to tpa.