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 ...)
I understand that a global agricultural market with a single norm would be better for mercantile exchanges, but for the single producer EU (or China, or whatever) is a sufficiently big market to be targeted exclusively if they want. OMC should not conflate normative barriers with tariff barriers, they have a real purpose (maybe USA is wrong to eat so much pesticides).
https://www.bloomberg.com/news/articles/2019-08-02/u-s-and-e...