That whole treason thing is a recent development.
No, it's not a new thing at all. Jefferson Davis was charged with treason after he was captured and was imprisoned for 2 years afterwards. He was never tried because he was pardoned before being brought to trial.
From Wikipedia: Davis was indicted for treason while imprisoned; one of his attorneys was ex-Governor Thomas Pratt of Maryland.[144] There was a great deal of discussion in 1865 about bringing treason trials, especially against Jefferson Davis. While there was no consensus in President Johnson's cabinet to do so, on June 11, 1866 the House of Representatives voted, 105-19, to support such a trial against Davis.
After two years of imprisonment, Davis was released on bail of $100,000, which was posted by prominent citizens including Horace Greeley, Cornelius Vanderbilt and Gerrit Smith. Davis remained under indictment until Andrew Johnson issued on Christmas Day of 1868 a presidential "pardon and amnesty" for the offense of treason to "every person who directly or indirectly participated in the late insurrection or rebellion" and after a federal circuit court on February 15, 1869, dismissed the case against Davis after the government's attorney informed the court that he would no longer continue to prosecute Davis.