I find this to be a very annoying thing that happens more often than WN seems to think it does. If you go to the employee intranet and go to in-flight ops, this is the first FAQ style question and it clearly says since they don't have a policy on seat saving either way, it is really up to the passengers to do whatever they want to do. I have heard some FAs say you cannot save exit rows seats, but that too is not a policy so you could even challenge them on that.
I think it would be better for FAs and passengers alike to have this clarified so that it is either more than fine to save seats or more than fine not to save seats. I can understand if there are plenty of seats available and it is not a prime location, but for the people who save the bulk heads or exit rows or entire rows and play reunite the couple, I just don't understand it.