Many of the justifications for moral decisions in Islam is supported by hadiths. There is a big debate at the moment over the actions of ISIS and whether it is even allowed. But my question why are hadiths used in islamic scholarship at all? The Bible has large amounts of Apocrypha that are not included in the official canon but contain stories of the disciples and jesus. How come Salafist who seek the most basic, 'authentic' form of Islam use texts that are not strictly the 'Word of God'?
A lot of biblical scholars will still look to the Apocrypha to understand the bible better. The other thing is, different sects and branches of christianity recognize books as being canonical that other churches do not. For example, Beta-Israel recognizes the book of Enoch as cannon, Catholics do not.
The same thing applies to Muslims. You say that they're using texts that aren't strictly the word of God, but to them it's exactly that. They have a collection Hadiths that they view to be cannon, that others don't necessarily agree with.