Because when one attacks a holy book of a certain religion for certain content, then it makes sense to check out if the holy books of other religions have similar content.

Sharia is not law in the sense that we normally think of as "law", ie statutes, regulations, ordinances, etc.  Sharia is a guide to one's relationship with God.  See, eg, http://www.dallasnews.com/opinion/commentary/2016/07/19/asifa-quraishi-landes-5-myths-shariah-law.