regarding International humanitarian law 

The protection of hospitals is according to the Geneva convention. The agreement from 1949 was also signed and ratified by the USA.
According to that agreement hospitals are protected.
The exception is: unless the facilities "are being used, outside their humanitarian function, to commit acts harmful to the enemy" as rightly commented in the article. But: Even if enemy combatants are inappropriately using the facility for shelter, the "rule of proportionality" usually forbids such attacks because of the high potential for civilian casualties. (why the usually as a qualifier?)
And it is not the: Human rights organization Human Rights Watch stated that the laws of war require the attacking force to issue a warning, and wait a reasonable time for a response, before attacking a medical unit being misused by combatants.
but the Geneva convention itself that requires a warning by the attacking force. And that is not a choice.
Why having qualifiers, as if the Geneva convention is not clear on the point of the protection of a hospital. And this scenario, an unannounced air strike by an AC-130 definitely falls outside a hypothetical proportional attack.
Why being so ambiguous about it, the Geneva conventions forbids the attack on a hospital, except in exceptional circumstances!