What is ironic about Pap's holding Huck against his will?

Although Huck's father is holding Huck against his will, Huck almost has more freedom then he does with Widow Douglas.  With his father Huck is allowed to smoke and curse.  Huck doesn't have to pretend to be civilized like he does with Widow Douglas.  Although Huck is able to return to his previous way of life, he still isn't happy and he wants to run away.