Wither Boss can break blocks! When I fired a arrow at him, he broke about a 3X3 area in my world "Second Try - Ocean World". Can you make terrain Wither-proof so the Wither Boss can't break blocks?

This is a Glitch! I Repeat! This is a Glitch! It's not a Bug, despite the reading up in the Issue Type section!

EDIT: RESOLVED!
Reason: (from [MOD] CubeTheThird) The Wither is supposed to destroy blocks. This would however be considered a bug if you had the game rule for mob griefing turned off, and this still occurred.