Why wasn't Harry sent to Azkaban for using "Cruciatus Curse"?

When he attempted it on Bellatrix in OOTP there were no other witnesses, and I can't really see her reporting it. Mocking or attacking back is more her style.

When he uses it on one of the Carrows in Deathly Hallows (can't remember which), McGonagall was the only witness. Again, she might have chosen not to report it.

cuz hes effing harry potter