Can't ban thermoses. They can be used to transport organs and the like and you could totally hide shit between the walls. Can't ban hams. That would gut the tourism industry because people bring food gifts home all the time. You can ban scary things, though. It's always good for some security theater.
They allow Lithium Ion Freaking batteries on the planes but, get this everyone reading this thread, You can carry them into the passenger compartment but not your checked luggage. Don't worry though, they will have new rules for LIon in cargo holds in place by 2018.
Lithium ion batteries are required for phones and other tech to work. Secondly, the "explosions" that these things make aren't anywhere near severe or dangerous enough to damage the plane or be worthy of death. As well, it can take quite some time to get one of the things to explode in a decent fashion so far as I am aware. Cargo holds likely experience a lot of depressurization, heat, cold, and other factors. these would likely damage or cause batteries to have issues and as a result the batteries should probably be kept in a fairly safe, controlled, environment.
The point of the rules is not to outright ban all things which can be dangerous at all, the point of the rules is to protect and ensure that nothing which is of imminent danger to an airplane is allowed onto the plane. People can cause havoc with lithium ion batteries. People can cause havoc with dry ice. The measily little pop and fire isn't going to kill people, and the smoke and warning the device gives off, the tampering required to get the battery to explode are all too difficult to cause substantial harm with. People aren't going to cause the instant death of 5 nearby people with a dry ice or a lithium ion explosion, and setting those things in motion is a very clumsy and hard to pull off sort of thing within the compartment of an aircraft. Again, and I cannot stress this enough, the people running the TSA and making policy decisions are experts at what they do. Despite that their decisions may not seem logical, I am almost certain that if you had the scope and knowledge of the TSA that the people who set these rules do then you would undoubtedly consider the policies relatively tame and reasonable.
And they aren't going to cause the instant death of anybody with a bowling trophy, either. Remember where this discussion started? Me pointing out that the TSA is crowing their safety record and celebrating their vigilance in confiscating a retirement gift from an NFL coach. Not "turned away" not "didn't let on the plane" but confiscated... and posted pictures on their blog... to show how safe they're making the world. Again, and I cannot stress this enough, these are the chucklefucks that spent $160m on pornoscanners. You can be "almost certain" all you want, they are incompetent buffoons that have not, by any applicable measure, accomplished a single fucking thing. http://www.politico.com/magazine/story/2014/01/tsa-screener-confession-102912People aren't going to cause the instant death of 5 nearby people with a dry ice or a lithium ion explosion, and setting those things in motion is a very clumsy and hard to pull off sort of thing within the compartment of an aircraft.