Why must some 'dangerous items' be in check-in baggage?
Generally, "dangerous" things that might be used as weapons are required to be kept out of the cabin. For instance, you can have firearms in checked baggage, but not carried in the cabin.

Given what's been learned about terrorist planning, there were plots to bring liquids on board an aircraft that could be combined to form an explosive. To counter that threat, the amounts &amp; types of liquids that can be brought on board have been limited. Since the liquids in isolation are safe, they're allowed in normal quantities in checked luggage.

In the case of Lithium Ion batteries, the concern is less their use as a weapon, and more about a defective battery catching fire. If this were to happen in a cargo hold, the fire would cause other things to start burning as well before the cargo smoke detectors alert the crew to what's going on. So the priority is to keep lithium ion batteries out of the cargo hold. They're allowed in the cabin (with some restrictions) since you're pretty likely to notice your cell phone getting hot &amp; smoking if it's in your pocket, and with that sort of early notice, the fire can be dealt with before it's caught large volumes of material on fire as well.

At least that's the theory. I don't know how many times it's been put to the test. In at least two cases (the Samsung Note 7 phone, and hover boards), devices which are considered to be at high risk of catching fire are simply kept off the aircraft altogether.