Libel law was changed by a New York jury, who voted to acquit printer John Peter Zenger of libel against the Royal Governor, William Cosby.

Up to that point, American colonial law kept step with British, and general European, law on the question of libel, which is to say, if someone speaks ill of another in print, it's libel. If the bad things that are said are true, that only makes the libel worse.

Zenger's defense attorney realized that, by law, his client was guilty of libel, but appealed directly to the jury in a strategy known as "jury nullification". He based his defense on the idea that people ought to be able to say the truth in print, good, bad or indifferent, and that libel only occurs when false statements damage the person's reputation. If true statements damage the person's reputation, that falls on the head of the person who committed the acts that support the truth of the statements, not on the reporter, publisher or printer.

When the jury in the Zenger case ignored the judge's instructions and voted to acquit, the libel law changed in New York colony. This change spread throughout the British colonies, which were increasingly more independent about the interpretation and enforcement of law among themselves.

Details on the Zenger case, which I find far more fascinating than what passes for courtroom drama lately, can be found here.

The American definition of libel is therefore very different from the original meaning, which is apparently still in force in Italy and other parts of Europe. As you say, it's an old European issue, with nothing to do with shari'a, though I can understand why Americans would think the two were connected. We are very used to our own definition of libel and think, incorrectly, that it's the world's standard. We forget our history.