I once suffered from PTSD (not related to military combat I better add,) and gaming is very good at desensitising horrific situations, and helping horrific memories become a just a distant and unpleasant memory. With games the consequences of dying, getting shot, mauled by zombies, crashing etc don't entirely matter in the grand scheme of things. Some of PTSD stems from our natural worries of our own mortality and the mortality of our friends and family, in games if something bad happens, it doesn't completely matter and isn't the end of the world, it's only pixels it can seem really trivial.
Gaming gives goals to concentrate on; getting to that next level, building that city, defeating that boss, improving our understanding of ACM, and in MMO games working together and interacting with other people etc. It keeps the mind occupied in complex thought and human interaction, unlike TV. TV and such can leave the mind swirling in its own thoughts without any break from bringing back those vivid memories one would hope to forget.
The mind is very good at remembering the good things you have experienced and MMO gaming is especially good at giving you new more pleasant memories to overwrite old ones you want to forget.