Fordan
Contributor
I don't know about the US, but in Britain I approached both my home insurer and several of the specialist travel insurers, and they all said that the only way they will cover against theft from a bag is if it is securely locked. Given that that is impossible for any flights on American airlines or routing via America, that means their insurance would not be valid. They were quite definite about it. They'll still gratefully accept the premiums of course, but they won't actually pay out.
You can lock checked bags in the US. You just have to use special locks that the TSA has master keys to. They're fairly common to buy within the US, although I'd assume a bit more challenging to find outside the US.
The place where you can run into trouble with locks is at Customs, and that isn't just a US thing. If a bag is delayed, it still needs to clear Customs, generally without you there. I've had an issue two times so far: coming into the US Customs just snipped the lock (they aren't the TSA with master keys), ziptied it after inspection, and sent it on, and Great Exuma in the Bahamas they sent a companion's bag to the hotel when it arrived on the last flight of the night, but I had to take a taxi back to the airport the next morning to claim mine and unlock the bag.