- Free trade is massively unpopular as a concept.
- Two parties (however defined) that implement a free trade agreement tend to see a rise in standard of living.
- Larger trade zones tend to be doing better along all kinds of metrics than smaller ones.
I think it's safe to say that free trade is in the public interest, and I think it's reasonable to consider public hatred for it as "a reason" to keep it secret. I can think all that without thinking that keeping it secret is a good idea.