"Generally" strongly assumes equal or even perfect competition.

For example say Country A and B trade coal and agree to no tariffs. Country A now cuts back on worker protection and wages, such that their coal is cheaper. Country B's only other chance than doing the same would be to raise a tariff to keep its own coal competitive. That's why Country B wants a say in Country A's labor laws.

And this is exactly the reason for a supra-national body that comes in with annoying regulations and red tape that stops you from competing with Johnny Foreigner in the full-blooded way you want to.

Let the race to the bottom commence!