[to1y]

>That's a bit like asking "why is pain after an injury a 'disorder'"?

It isn't. If anything that supports OPs point

[xboxnolifes]

Disorder doesn't mean an unexpected outcome. It's a disruption in the norm for the system. The stable state for a human body is not constant pain.

[tsimionescu]

I should have said (and I believe explained in the rest of the comment) that it's a bit like asking "why is pain after an injury a 'disorder'" of a person suffering from chronic pain. PTSD is a disorder beacause it is a chronic version of a completely normal accute reaction.