Why do فقهاء (Islamic jurists) use verses that indicate the prohibition of major sins (such as shirk and delving into Allah's verses) to prove the prohibition of lesser sins (such as disobeying parents in matters not involving shirk, and sitting with an alcoholic or a music listener)?

Religious rulings are inferred by understanding evidence such as the Qur'an, Sunnah (definition), Ijma' (consensus), and Qiyas (definition) (analogy). Among the types of definitive Qiyas is: when the unmentioned case is more deserving of the ruling than the explicitly mentioned case, and when it is equal to it. This is called Mafhum al-Muwafaqah (concordant understanding), and it has two types: Mafhum Awlawi (prioritizing understanding), such as the prohibition of striking being analogized from the prohibition of saying "uff" (expressing displeasure); and Mafhum Musawi (equal understanding), such as the prohibition of burning an orphan's money being analogized from the prohibition of consuming it. As for the lower Mafhum al-Muwafaqah (where what is mentioned in the verse is superior to what is unmentioned), it is not considered proof by consensus, but other evidence may exist to establish the same ruling, such as disobeying parents in sin, and not sitting with a wine-drinker.