I have absolutely no brief for Joyce. He was an odious figure well before the war. However, although brought up in the US and a naturalised German, he was Irish by birth. As such he did not owe allegiance to the Crown; he therefore could not be guilty of Treason.
However, the Courts could not let him go free since public opinion wanted to see him executed.
What eventually brought Joyce down was the legal fiction that as he left UK for Germany in 1939 with a forged British passport given to him by the German embassy in London he obtained the protection of the crown. As every lawyer knows, a forged document is a nullity, so the execution was fixed. In legal terms this is unacceptable although his acquittal on legal argument would have been unacceptable to the public at that time!