I initially thought it was, then the generic Off Peak page on NRE was changed to imply otherwise.
If it is a a deliberate change then, other than to maximise the last 0.0000001% of revenue or out of sheer bloody mindedness, I'm struggling to think of the reason why the rail industry would want to do this.
To stop people splitting tickets or starting/finishing short? A BoJ restriction on just the outward portion should do that.
To force the tiny proportion if people who want to break their return journey to buy more expensive SORs instead of SVRs?
As the first step in a secret plan to erode passenger rights get rid of BoJ entirely by first changing 8A tickets, then if nobody kicks up a fuss, gradually changing all the rest?
BoJ is one of the good things about rail travel. Being able to stop off on the way one of the advantages that the train (and the car) has over the bus and the plane. It should be something that the rail industry is promoting, not trying to abolish. In an ideal world I'd allow unrestricted BoJ on all walk up tickets, on both the outward and return legs. A tiny proportion of people may use that to save a few quid by starting or finishing short on a a less expensive ticket, but that's a problem of the rail industry's own making caused by price gouging on popular routes.