I think that's exactly why circus life is so appealing to some. The circus and carnival represent freedom on a couple of fronts. First, particularly for the character, the circus is his freedom from his family. He no longer has to deal with them if he doesn't want to. He has control over the situation now, where he didn't before.Duke's plunge into circus life reminded me of the Russian literary critic Mikhail Bakhtin who saw the carnival and carnivalesque as a celebration of freedom, a means of escaping social norms and conventions. At the circus, we are all ready to embrace the carnival atmosphere and forget restrictions.
Second, the idea of the circus or carnival as "a means of escaping social norms and conventions" is absolutely correct. When you think circus, what do you picture? I picture animal performances and clowns to start, but thinking back on older versions of the circus, I picture "freaks" and the abnormal things they could do for performances. The Bearded Lady, the Strong Man, and any others (though some were absolutely manufactured "freaks") all represent a break from the normal. The circus has always been a safe-harbor of sorts for "freaks." Circuses built themselves up on a foundation of abnormal, so it's really no wonder why people would see them as a point of freedom.
Third (I know, I said only a couple points), the circus or carnival could have been a means of financial freedom for some. Granted, money depended on customer attendance, but think about it. You don't need a formal education to work in the circus. If you could perform manual labor of some kind or create an act that would entertain audiences, you were pretty much set for circus work. That could potentially make the circus a haven for folks who couldn't afford education or formal training in a trade. Now, this may not be true in modern circuses, and may absolutely be an outdated point of view. I was thinking more in past on this particular point.