The doctor can fix it but that doesn't mean that boxing isn't dangerous
If the doctor can fix it then what's the "danger?"
If the EMH's objection is to any
activity the could possibly produce an injury, then that covers quite a bit of ground. We saw Picard riding a horse at a canter, Crusher (iirc) once treated a crewmember who broke their arm playing parrises squares.
just because an injury can be healed doesn't mean an activity can't be dangerous. A potential for injury is what makes something dangerous.