Huck's delightful "Pap" is attempting, through any means he can, to get Huck's six thousand dollars. He has been trying to go through the courts to get it, but Judge Thatcher is using the law to postpone the trial for as long as he can; Thatcher is also trying to help Huck get adopted by the widow so that Pap can't get the money or bother Huck anymore. Pap is upset that it is taking so long. He says, "Here's the law a-standing ready to take a man's son away from him-a man's own son, which he has had all the trouble and all the anxiety and all th expense of raising....And they call that govment!" The amusement here is that Pap never took care of Huck at all, and he's acting all indignant that they want to take Huck away. He goes on to complain that "the law takes a man worth six thousand dollars and up'ards, and jams him into an old trap of a cabin like this". He's just upset he's not getting the money; so, it's the government's fault.
He also thinks it's awful that the government would let a mulatto vote, hold a job, and wear nice clothes. Because of this, he vows that "I'll never vote ag'in." Huck says that whenever Pap is drunk he tends to rant and rail against the government, and he probably just finds new reasons each time.