Also the beta isn't tied to the paid subscription. Thus it will be available tom some (for free) even without owning the PS3 version.

Instead I am puzzled by this "one way" thing. What about if one wants to play in both PS4 and PS3 with the same character? Just think about people living in two different places (students for example). I don't see many reasons why SE after upgrading to PS4 shouldn't let people play on PS3 as well.