In Norway, there is a large import tax on foods like cheese and meat, to encourage buying locally. This seems to work, as most shops only sell Norwegian produce.
This is also motivated by local food monopolies where they don't want competition from cheaper and higher quality cheese producers in the rest of Europe.
It's certainly maintained by that, but Norway has a couple of centuries history of political focus on food security, ever since the British naval blockade of Denmark-Norway during the Napoleonic wars, and then strongly reinforced by the nazi occupation. The strong focus on keeping the rural areas settled also in large part stems from that, though of course it is also self-reinforcing in that people who now benefit from policies designed to do so tend to want it to continue for their own reasons too.
There's a lot of cultural significance of food security, going back to e.g. decades of making primary school children learn about Terje Vigen (Ibsen's epic poem about someone trying to brave the blockade to feed his family), coupled with a lot of cold war thinking that at least up to the end of the 80's saw food security as part national defence during a time where we still had air raid siren tests many times a year in case of Soviet invasion.
While that has certainly softened up since, most Norwegian politicians still grew up with that.
The EEA Agreement provides for a free trade area covering all the EEA States. However, the EEA
Agreement does not extend the EU Customs Union to the EEA EFTA States. The aim of both the free
trade area and the EU Customs Union is to abolish tariffs on trade between the parties. However,
whereas in the EU Customs Union, the EU Member States have abolished customs borders and
procedures between each other, these are still in place in trade between the EEA EFTA States and the
EU, as well as in trade between the three EEA EFTA States. Furthermore, the common customs tariff
on imports to the EU from third countries is not harmonised with the customs tariffs of the EEA EFTA
States