We ran a series of CQB games this winter here in Kitchener. Under 350fps, mercy optional, full mask optional, auto allowed. No injuries other than welts.
As to why not mandatory full mask? Same reason why it's not mandatory at outdoor fields even though the guns are even hotter: you can't bring most guns up to your face with a full mask on. I got enough hits to the face that I eventually made a cut-down mask just to protect the nose and mouth, but I still wouldn't use a full mask for CQB. Teeths can be replaced, eyes can't.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."