He wants to have his cake and eat it. He can copyright his work to have it protected, but won't because that would mean you know the name of the person who makes it. So instead he abuses the law by claiming a trademark - which he isn't entitled to under the law. So now someone has challenged his trademark and because Banksy isn't actually some anti-establishment rebel, he's using fancy lawyers and public pressure to get his way despite not actually being right as a matter of law.