tnt does damage through blocks it cant break

tnt does damage through blocks like obsidian and bedrock
steps to reproduce:
1 build a box of obsidian or bedrock just large enough to fit a player inside (fill in corners and edges too)
2 surround the box in tnt
3 put a mob or player inside
4 ignite the tnt
5 the mob or player will take damage through the obsidian/bedrock