That's right, Brent. They are legal
to carry on board SUBJECT TO THE DISCRETION OF THE INSPECTOR and if the inspector says "NO" you have to throw them out or find a way to send them home. Some airlines/airports will hold them for you to pickup on return, if you put them in a marked envelope. Others won't.
It could be an innocent CO2 cannister designed to save your life, or a sealed cannister with Sarin gas so you can kill everyone on the plane and crash it over a populated city. There's no way to tell without opening it, so many inspectors will say "NO!" and their site supervisors will ALWAYS back them up 100%, no discussion.
The airline can accept it. The TSA does accept it. The FAA allows it. The guy in the inspection line overrides them all, at his/her own discretion or ignorance.
You could always try going to your departure airport
(in person) and asking the TSA site supervisor for advance permission but of course, that doesn't get it home either.
In checked baggage, it is more likely to be passed BUT you know the TSA routinely tampers with bags and there is a chance they will either remove it, trip it, or pull everything off the plane while investigating.
The laws are clear: Its a crapshoot, depending on which petty hireling thinks they are saving the world today.