The relevant rule is "When a player seeks out an opponent and throws an elbow which makes any contact above the shoulders, a Flagrant Foul Penalty 2 will be assessed."
"Seeks out" can be a slippery term and is the closest thing to assessing intent in the flagrant rules but Melo definitely jumped at Nurkic (not unusual), led with his elbow (unusual), and caught him in the head. Pretty much right in line with the rulebook.
People seem upset because it didn't "feel" like a Flagrant 2, either because wanting/thinking an intent to injure needs to be present, or because they believe others have gotten away with similar actions. Either one could be an argument for changing the rulebook, but both are flimsy criticisms of how it was applied.