I dare say that if the regulations are the reason why, it isn't becuase the Dept of Energy said no, but becuase SBR didn't want to fill out paper work.
I am not sure that this is true. From what I have read (take with a grain of salt) tritium glow items can be imported for "non-trivial" uses. So a keychain would be a trivial use. Firearm sights are a ligitimate use.
I cannot confirm or deny this, but this sort of rule sounds like something a guy in Washington would make up. And if you go under the assumption that radioactive material is bad (true in many, but not all cases), then this rule makes sense.