It's revenue protection, so that you can't just rely on the upgrade vouchers to get into the J cabin - otherwise everyone would just buy the cheapest Y ticket they could, and we'd end up with a system like the US where Elites flood the F cabin for nothing (although there's some efforts from the US airlines to change this and move closer to the European model).
There's a discussion on almost exactly this point over at the BA forum:
http://www.flyertalk.com/forum/briti...st-failed.html
So A3 is not exactly unique in Europe in managing its seat inventory this way.
In other words, if you want to guarantee J, you have to pay for it.