I always thought the ease with which injuries could be fixed in the magical world made them much more blase about injuries. Children can and do get very serious injuries playing Quidditch but even when all your bones disappear from your arm, just one night in the hospital wing can fix it so you're good as new. If you can easily relieve the victim's pain and easily fix the victim's injury so it's as if it never happened, the cause of the injury in the first place becomes less important and maybe even less worthy of reproach. (Of course, I don't think this entirely addresses the fact that in your example, one student was about to inflict pain on purpose with an Unforgivable and IIRC Sectumsempra actually has long-term effects that aren't easily fixed by magic, but I think that the easily-fixed magical injuries inform the underlying attitude the educators have about putting and keeping their students in danger.)