> it was transparently obvious that the idea of minimum spending commitment to NATO was intended to prop up the US arms industry

...to Trump. European leaders took it literally: since the USA stopped being a reliable partner, Europe needs to depend on itself for protection. It makes zero sense to buy American weapons if you can produce/purchase them on the continent.

They knew what Trump meant, but this way they could agree at a surface level to keep him happy, while actively distancing themselves in reality.