As you point out ladybirder, the social pressures on these children are the root of the issue here.
Equating violence with masculinity has gone on since time began, and in some areas of today's society, men wish to display their own 'pride' by being seen to raise children who are though enough to fight and win.
The pressure on the child is therefore twofold - a fear of being seen as 'soft' by his peer group, combined with a feeling of 'letting down' his father by not coming up to perceived expectations. And that's before we get into the physical dangers involved in this type of activity.
As always, education is at the root of the solution. Children should be learning that being pysically stronger and more brutal than other children is not a badge of superiority, and this culture of violence should be outlawed.
The defence by one father, that his son would be 'throwing stones at buses' were he not busily engaged in inflicting physical damage on another child for the edificiation of adults who should know better, really says it all. These people really believe that this activity is preferable to what they perceive as the only alternative - vandalism and anti-social behaviour.
This is wrong on so many levels - as in fact is boxing, for all its regulation and 'protection'/ A sport where the stated aim is the brain damage (hopefully temporary) necessary to render an opponent unconcious has to have no placec in a civilsed society.
This activity does not even have that iffy justification - there is no safety code, it is visceral violence involving children as entertainment, and it should be banned. Now.