Imagine that you're working on some project and you find one of those forks with no GPLv3 attached (or worse, some other license that is more permissive). You integrate it, publish, and then find out that your project is in copyright violation.
I'd rather see a takedown induce an easily remediated repo change, than a big legal PITA down the road. It's not like he took his repo down, nor any of the forks that kept the license. He just DMCA'd the ones that stripped the license files.
But, the original author did give that option.
It would occur to me, and I might not be strong enough to resist.
PONIES!
ETA: An HN account of the same name was created 235 days ago, so it's probably the same person if they choose to respond.