I think the idea of free trade is that more wealth is made through comparative advantages and trade, meaning everyone contributes less "blood and backs" and gets more wealth in return [1]. Not sure how this applies to military assistance, but it seems likely Japan and EU would not have the peace dividend they've had since WW2 if US wasn't subsidizing their defense.

1. https://www.khanacademy.org/economics-finance-domain/ap-macr...