Tnt cart doesn't drop tnt

If you punch a minecart with TNT, it won't drop the tnt.

What I expected to happen was:
The tnt and the minecart would be dropped if I punched the minecart.

What actually happened was:
Only the minecart is dropped, the TNT is not dropped.

How to reproduce:

1. Place a rail (any type) on the ground.
2. Place the TNT cart on the rail.
3. Punch the TNT cart until it's destroyed.
4. Notice that only the cart is dropped, and not the tnt