Apparently it was close air support. A Joint Terminal Attack Controller attached to US special forces was on the ground. Afghans were apparently under attack from the area of the hospital (though no one has said "from the hospital"). Even if they were - hypothetically - under attack directly from the hospital, self defense against an attack from a medical facility would need to be proportional. The amount of casualties (22 dead and 37 wounded) seems very high. I think US commanders are very much aware of this fact, and now state the attack was a mistake.
They are in a war zone, and that carries with it risks, be it of accidental or deliberate cases of things blowing up nearby
True. These medical professionals are very much aware of the risks, probably more than most, they see the consequences on a daily basis. They volunteered to help, and they made a calculated decision to take some risks. Probably they didn't expect a US attack on this scale, and I don't think they should have reason to expect that.
Those who are responsible for the decisions that led to this mistake should also be aware of the risks to them, even if they're small in comparison. Unfortunately, it's exceptionally rare for a country to try it's own soldiers for war crimes (deliberately attacking a medical facility is a war crime). I think certain standards of humanity should be upheld, and thorough investigation that isn't a cover-up seems appropriate. I'm not holding my breath for it to happen though