Why is Huck afraid of being called an abolitionist? Why is this ironic?

The word abolitionist is despised in the South, where most people own slaves. This is ironic because Huck, is in fact, actiing as an abolitionist in his desire to aid Jim in his journey to freedom.

“Well, I did. I said I wouldn’t, and I’ll stick to it. Honest injun, I will. People would call me a low-down Abolitionist and despise me for keeping mum—but that don’t make no difference. I ain’t a-going to tell, and I ain’t a-going back there, anyways. So, now, le’s know all about it.”