The difference is that while they can restrict the what, they can't restrict the how. Yeah they could make training LLMs illegal, but they can't for example put a quota on how much training you can do. Passing a law to ban something completely is a lot harder than passing a law that puts a "minor restriction" in place.
Ultimately any law can be repealed, so the loophole of changing the law in the future always exists. The point is that any future change to the law will take time and effort, so people can be confident in the near term that they won't be subject to the whims of a regulator or judge making decisions in a legal grayzone which may come down to which side of the bed they woke up on.
Ultimately any law can be repealed, so the loophole of changing the law in the future always exists. The point is that any future change to the law will take time and effort, so people can be confident in the near term that they won't be subject to the whims of a regulator or judge making decisions in a legal grayzone which may come down to which side of the bed they woke up on.