I don't think it's just your son's team that doesn't understand it. I'm sure there must be ways of utilising a forward standing near the goal line ('goal-hanging') to receive a long throw, especially if the defending team are geared to taking a high line. I'm surprised someone hasn't set up in...
And why not? A 40 - 50 yard flat trajectory throw could be a potent weapon, especially as (I think I'm right here) you can't be offside direct from a throw in.