I believe the reason is that if they sell are expecting less than 80-85% (Figure I cannot remember) of the tickets then regulations are ok for sit where you want. If they sell a high percentage of the tickets then regulations mean the seating has to be allocated for safety purposes. (Im sure I read this a few years ago when we were back in the Vanarama and people were complaining about not being be able to pay on the day or sit where they want.
For Example Wycombe would have been allocated seating but as we sold a lowish % of the seats available they were able to change to unallocated.