All a bit mad, isn't it? I recently had to send a battery unit away for repair. The T&Cs for the courier recommended by the repairer said they would not take a "battery" but would take a "controller" with a built in battery. The only justification I can think of is that the latter assumes some protection circuit but how can they, or anyone tell?
Anyway, I bagged and double boxed it and it got there and back (within UK, surface transport only) safely and has not as far as I can tell been stickered, X-rayed or otherwise examined.
Meanwhile I have ordered many batteries and battery containing gadgets online which have been delivered by post and the various courier firms without fuss. Maybe professional sellers have some special conditions which means they are trusted to pack properly, or maybe they just don't declare.
I'm sure accidents do happen, but it would appear to be self evident by the number of non-combusting delivery trucks that the risk isn't as big as somebodies safety consultants thought it was.