Unbreaking Enchant on Bow/Sword/Armor

When enchanting a weapon/armor or picking up an enchanted drop from a slain mob, the Unbreaking enchant is commonly found.

What I expected to happen was...:
Unbreaking shouldn't be an available enchant on certain things like Bows & Swords, if I remember enchanting mechanics correctly.

What actually happened was...:
At a level 30 enchant of a bow, I received an Unbreaking III bow. Another level 30 enchant of a diamond sword resulted with Unbreaking III, Bane of Arthropods IV. A level 22 enchant of Diamond Boots gave me Protection III, Unbreaking III.

Steps to Reproduce:
1. Enchant a piece of armor or a sword or a bow.
2. There is a (good?) chance of Unbreaking showing up

P.S. There may be more things bugged with enchanting, but the Unbreaking is the most obvious and easiest to evidence.