I think it is just the airline.
My cousin's husband was on an archeological dig in Israel and brought home an old axe head. They were very clear it MUST go in check in luggage.

I've flown with sword domestically at least half a dozen times. I've seen them checked in with the hilt still hanging out of the bag. And, that was less than 6 months after 911.

So, it must just be this airline.
Maybe because folks have tried to steal objects and fly home with them.
That's as good of a reason as any I've heard.