Speculating (I am not a lawyer) I see two options:

1. Model weights are the output of mathematical principles, in the US facts are not copyrightable, so in general math is not copyrightable.

2. Model weights are the derivative work of all copyrighted works it was trained on - in which case, it would be similar to creating a new picture which contains every other picture in the world inside of it. Who is the copyright owner? Well, everyone, since it includes so many other copyright holders' works in it.