He probably saw them as weak, and not being able to pull off his plan. Joey seemed like a good kid, that wouldn't want to hurt anybody, so he wouldn't join the Injustice Society. And William seemed like a father who loved his son, and wouldn't have forced his son to join the Society, if his son didn't want to. William seemed like he would side with his son over the Injustice Society. When Joey showed him his trick, his reaction was to encourage him and give him the pointer of misdirection. There was nothing to show that he was embarrassed or disappointed that his son wasn't as good as he was, or anything but a good kid.
William seemed to be all about his family life. He also didn't call Icicle after he found out somebody had the Cosmic Staff. His heart wasn't completely in the Injustice Society.