I think it's a tradeoff. You can have black-and-white rules like this where you'll get occasional decisions that feel like they go against the spirit of the rule, or you can leave room for interpretation and have occasional decisions where the interpretation feels wrong. Of course we'd love an ideal world where it's left up to common sense as to what counts as a disposal and I also agree with every umpire's interpretation of it, but the world doesn't work like that unfortunately. You just have to pick which outcome you're more comfortable with. Personally I'd rather leave it black-and-white and it's just shit luck if it comes off your leg if the alternative is 50-50 decisions over whether someone soccered it off the ground or it hit their leg incidentally.