It seems that they get the repair spells, logically - if you have the arcane ability to help something, you probably also have the arcane ability to destroy it. (Why I am using the word logical, and the word arcane in the same sentence I don't know).
Was there a rationale? Or is it just that the House Cannith quests - where destroying constructs became very important - came out after the Wizard spell set. Is it s game balance thing?