The argument is specious and starts from questionable assumptions. I see two of them.
1/ The defence of these countries would be expensive and would only benefit them.
No, it’s to be put into perspective. The countries thus “protected” keep an army and buy mainly American, it’s a win-win situation. In fact, Trump has put pressure on NATO member countries to buy even more US armaments. The process is so forced that France protested via its defence minister who said
This makes the USA, among other things, by far the world’s largest arms manufacturer and seller.
Does it really cost the USA to ensure this defence? I’m not sure, it allows them to maintain vassals and an advantageous hegemony. Are the protected countries really saving by being under Uncle Sam’s thumb? That remains to be proven, and it depends on the country. France retains military independence while having social security. We could very well do without NATO while keeping our social security.
2/ The USA would therefore be forced to adopt a liberal system that would cost less.
NOTHING justifies the continuation of such a bad system except to continue the benefits of a few, including American doctors .