It's TSA but it's mostly safety and common sense as the fires produced by LiPo's aren't very easy to extinguish and on an airplane you have limited resources to fight fires. Once your extinguishers are used up you're out of luck. In a cargo hold, the fire fighting and suppression systems are also limited in what they can accomplish. LiPo's just happen to be very high in energy density and very persistent when they ignite, very bad for the inside of an airliner.
Please discharge the packs as much as possible before flying with them and make sure they are packed safely to avoid being damaged during the flight.
The passengers and flight crew will appreciate it very much!