What is ironic about Tom leaving the Cadets? Chapter 22

Tom's quitting the Cadets was ironic because he'd left over the things he couldn't do, but once he left, he found he no longer wanted to do them.

Tom was a free boy again, however—there was something in that. He could drink and swear, now—but found to his surprise that he did not want to. The simple fact that he could, took the desire away, and the charm of it.