Add assists for being in close proximity of a tower going down

I'm kind of curious why being in close proximity of an enemy tower being destroyed doesnt give an assist unless you actually hit it. Let me give you two examples:

1. Denying farm. You dont hit the tower because you want to deny as much farm as possible to the enemy, so you let the minions take it. I think this is a great strategy and I don't understand why you are punished for doing so.

2. You are a melee champ and your team is sieging a tower. You are positioned between the tower and your ranged carries that are attacking it. There are enemies there and you are threatening the dive with your positioning which zones the enemy team from attacking your ranged carries or wave clearing.

In both scenarios you are actively helping to destroy the tower, but you are denied the gold and objective participation toward your grade.

Solution: if you maintain close proximity to a tower for 4 seconds before it goes down, you get credited with the assist and gold. I think that would prevent people from just walking by for the assist and seems fair. I don't think its right to punish people because they aren't actively attacking the tower, but are still actively helping to take the tower.