区别在于：

①意思不同

on free意思是自由的，免费的的意思，而for free只能是免费的的意思

Willy Lin has cut back on free drumsticks in the canteen of his Jiangxi clothing factory as meat and vegetables grow dear.由于肉价和菜价的不断攀升，林威利削减了他的江西服装厂食堂里的免费鸡腿。

②说免费的通常不用on free，free通常很少与on搭配，for free是固定搭配

Children lining up for free food.

孩子们排队领取免费食物。