seems like a risky move for the yeast companies if they do that but you right that technically it would not be that hard for them to do. i'm just thinking they might get lawyered.
I don't think there is anything that a brewery could do as long as the yeast company doesn't advertise where it came from. If you look at the yeast strain chart, I doubt all of those breweries voluntarily gave up their yeast. I'd bet that the new small yeast companies harvested some of their products directly from white labs/wyeast vials.
https://www.mrmalty.com/yeast.htm