How exactly is a company supposed to prevent a contracted writer from doing something like this?

If you contract someone to do something you are still responsible for the final product. Even IF their contractor was entirely at fault, they had a responsibility to double check the work. And this wasn't some hard to realize plagiarism. It was blatant word for word lifting, from something that is fairly recent and should have been well known about in the circles Bethesda operates in.

Don't excuse Bethesda for shitty quality control. While this is certainly different than the F76 disaster and many of their other recent missteps, they are ALL heavily influenced by quality control, and an inability for the company to double check work before rushing it out for a quick buck.