I think the idea is that culling a user is something that happens a lot, even during normal operations. As a company running a hub you might cull inactive users every 16h or faster and active users after 5days or so. This means you want to disconnect deleting a user and their PVs.
The other thought was that deleting a PV is very final. If you did that by accident it is very unlikely that you could revert it. So better to make it a manual step.
I think having a flag that allows DELETE /api/user/<id> to tell a spawner that it should run clean up tasks for that user would be an interesting idea. Then kubespawner could offer to clean up PVs, though I think we should keep it turned off by default and put a big warning label on the option in kubespawner.