Well, a trademark has to be used as a trademark, you can't trademark things as a means of throwing a towel on a deckchair that might want to use later.
Copyright can be assigned to another entity (person or company), so he could have assigned his copyright to, say, his agent. But otherwise, copyright belongs to an identity (that's by definition) identifiable, so if you opt not to be identifiable, it's not surprising that copyright can't be assigned. Anyone could stand up and declare, Spartacus-like, that I'm the owner. The second issue is that by sticking his artwork in a public place, often on space that doesn't belong to him, that his intention is to promulgate.
I'm not really clear why they challenged it.