Why is open trade a bad thing?
Here's my take on the pros and cons as a layman. I'm very willing to be corrected by someone more experienced in these matters.
One of the challenges with free trade is that production of goods and services will tend to be outsourced to places that have the lowest overhead. That's fine in and of itself, that's how capitalism works - but oftentimes those locations achieve that low overhead by skimping on worker safety/rights, allowing child labor, ignoring pollution, and other unfair or unethical practices. They may suffer from corruption that makes it hard to stamp out those practices (regulatory organizations can be easily paid off, etc.).
In an ideal world the consumers buying the goods and services produced in this way would realize that their cheap goods are being subsidized by these externalities, and make more informed choices (perhaps opting to buy more expensive but more ethically sound products). In reality, many (most?) people do not, and instead make purchasing decisions based on the immediate price/utility trade offs they can see. Trade regulations can allow a government to put its thumb on the scale and force some of these externalities to be better represented in the price... but of course they are used for all sorts of other reasons (protectionism, regulatory capture, economic pressure on rivals) that may or may not be a positive for the consumers or other parties involved.
Getting the balance right is tricky, but I think that truly free trade is an ideal that doesn't play out well in the real world. It could work fine in a world with relatively standardized environmental and worker regulations across countries, but we do not live in such a world.