I would be very surprised if they had a contract that would allow it. It’s certainly possible but they would have really bad lawyers or really bad negotiators to allow for that.
Never attribute to malice that which can be explained by incompetence.
But, likewise, never attribute to incompetence that which can be explained by apathy.
I have worked on projects with contracts like this. All that would have to happen is that Bethesda was internally convinced the Fallout name was a necessary part of success. "You want the code for some unrelated game? Have at it, just don't release anything more recent than two years old." Then the new game manages to get the Westworld name and Bethesda suddenly realizes that's even bigger.
It doesn't require bad lawyers or bad negotiators. It just requires a difference in perceived value of code vs. trademarks, and the other party being proved right later.
KEEP IN MIND I'm not saying this is what happened. I haven't seen the contract. But we shouldn't throw around terms like "theft" until it's at least been read by someone who can comment on it.