Infinite use of tools

What I expected to happen was...:
When I switch inventory slots in the middle of chopping a tree down, the mining damage would reset.

What actually happened was...:
When I switched inventory slots the mining damage to the block did not reset.

Problem:
Basically, people can use diamond axe/pickaxe/shovel to mostly mine a block then switch the inventory slots to something else. When this is done, The mining damage to the block continues and players can break anything with essentially their hands.

Steps to Reproduce:
1. Go to a tree
2. Have an axe in your active inventory slot
3. Start to break a wood block
4. When you see that the wood block is almost destroyed, switch your active inventory slot to something else. You will then break the wood without damaging your pickaxe.