Mark Twain did not like copyright in the U.S. He thought it wasn't favorable enough for authors. In his defense, Canada's lax copyright laws in the late-19th century allowed many pirated books to be sold cheaply and easily in the U.S., with the author not seeing any of the profit. This is, in fact, what happened with Tom Sawyer, which was published in Canada - before being published in the States - in 1876.
Twain never saw as much money as he would have liked from the publication of Tom Sawyer (he didn't see as much money as would have he liked from the publication of pretty much all of his books), and he blamed the poor sales figures (poor sales figures in his eyes, anyway) largely on the lax copyright laws in Canada and enforcement laws in the U.S, as this letter illustrates.
Twain never got over his hatred of the system of copyright as it existed in America, leading him to write an inane letter to congress near the end of his life (published as an essay called "Concerning Copyright", which can be found in this wonderful volume of Twain's later writings), detailing his own modest proposal: allow works to NEVER go out of copyright, but, assuming they are still published by the author or his/her heirs, decree that there must be a copy of the work available for 25 cents.
I wonder how Twain would feel about the current state of copyright: Life plus 70 years, a far generous sum by any measure. (One wonders how the founders would jibe the current duration of copyright with the constitutional provision that copyright should exist "for limited times".)