Because teachers have rotating lunch duties because it's often against their contract to make them work more than x lunches per week/month, while the child can be arbitrarily put in punishment for more than that.

Just because he was aware of the punishment doesn't make the punishment appropriate. At some point the issue should be escalated or a compromise reached. Allowing to it go on for the majority of the school year is the teacher being unable to control the situation so taking it out on the student to the students detriment instead of benefit.