Lol the question should be "why do we care if boxers smoke weed"?
But nah Phook, I feel ya - it seems that they know this is counted as a positive test, and can have massive consequences, and repercussions. . . So I really don't know why you'd risk smoking weed anywhere NEAR the time of your fight.
BUt as far as smoking weed during training camp, I would say that it hugely assists in relaxing the boxers after hard days of training, which is a downright brutal assault on the mind, and body for 8-12 week period.
SLeep, especially deep sleep for 8-10 hours is just as crucial as training, and diet in recovery of muscles (not to mention the mind as well).
So I imagine some boxers enjoy smoking some weed at night after a long day of training to help them unwind, and get some nice, sound, deep sleep. . .