All entities in the games (except items) damage tools even if you don't cause damage to them.
Example:
Rapidly hit a zombie with a sword, you will notice that the sword's durability has gone down a lot more than the zombie got damaged.
MC-17954
ALL non-item entities damage tools
Resolved
Duplicate
0
user-4968d
(Unassigned)
Unconfirmed
Minecraft 1.5.1, Minecraft 1.5.2, Snapshot 13w23b, Snapshot 13w24a, Snapshot 13w24b
Retrieved
Duplicate of MC-4732, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being closed as duplicate.