There are two subsets of that group who tend to oppose open weights:

1) The people who think LLMs are "inchoate super-intelligences", but then your argument is entirely correct and they should be happier to see less exuberant development of the things than to make sure the rapid advancement of the world-destroying technology is locked up inside corporate entities that are themselves poorly-aligned. So these people either don't actually exist or are making an honest mistake and should update.

2) Disingenuous sock puppets who pretend to be the first group because they want regulatory capture and know that "we need to regulate this because it's unsafe" sells more tickets than "we need to regulate this to exclude competitors and drive up margins".