It's also a weird contradiction that I've seen in practice now.
So I used to help run a community radio station, and we had a couple of major volunteers who were on JSA - so they were limited to 16 hours a week (well, 16 hours that they knew of), and were routinely told to spend less time doing the stuff. But the Job Center was happy to send loads of people our way to do the 6 week, 30hour ish placements. Seems weird enough, and even moreso when it's often the ones who were getting told off for the job center for for doing stuff there who'd be helping train people the job center sent there. And despite many interventions, they refused to think this was weird.
That example aside, I think it's definitely fair to say there's some oddities in the work ethics they encourage. Initially, oh no, no more than 16 hours volunteering, don't do that. You hit six months, then it's all why aren't you spending all your time volunteering, that's the solution. I get that they want to make sure you spend time jobhunting more than just being on JSA as a volunteering wage, but still - it seems a weird limit.
If you think getting people doing stuff like volunteering is good, encourage 'em earlier rather than have a limit that is only removed when people are forced in to it, surely?