I guess it comes to supply and demand. Other countries want US technology and services. They don't want US agricultural products as eluded to in the above thread. Thus, trying to sell stuff others don't want ends up making more noise compared to selling stuff they do want.

The US does this because: 1. National security (US farmers wouldn't grow stuff if nobody bought it, which is a problem if the US ever goes to war. Then, they might suddenly need it.) 2. Vote banks (Rural US happens to be the key in deciding elections and support for laws. Due to the electoral college and each state having 2 senators.) 3. Ideology (The reasons you mentioned. Free market ...)