Obama's apology for hospital bombing

US President Obama has apologised to the head of Doctors Without Borders, also known as MSF (Medecins Sans Frontieres), for the deadly US bombing on the Afghan hospital. But he offered only an apology, not an explanation.

A mere apology for 22 lives! Do human lives matter so little? Hasn't the US military committed a war crime by bombing a hospital and killing so many people?
Ashek Sarker
Comilla