Why should food being transported further be subsidized more than locally sourced food?

Local food often has higher transportation costs than food from further away. Long distance food transportation is done by highly efficient ship & rail. The last mile transportation costs typically dominate. And semi-trailer from warehouse to grocery store is far more efficient than half-ton truck from farmer to farmer's market.

And then both are dominated by the drive from the grocery store to your house. If you care about food carbon emissions, walk/bike to your grocery store. That'll have far more impact than food choice.