I've shipped my bike FedEx in its hard case at least ten times. No problems. Even if safety were equal, there's that whole "not having to schlep an enormous 50# suitcase through an airport and having to hire a van instead of a regular taxi" thing.
Freight haulers may also open containers, though obviously it's less common than it is with the TSA. DH and I have both had situations where our cases have arrived with the fastenings loose - my bike once from FedEx, his camera gear once on the airlines. Thankfully everything was still secure inside, but the lesson there is to pack everything as much as possible so that it won't rattle even if the outer fastenings are loose. After that experience, I sharpie'd arrows on the outside of my case pointing to the straps with the words "Please re-tighten all straps if opened."
(I also sharpie'd my name, cell # and email address on the inside of all my luggage and shipping cases. A piece of paper with that information is just as likely to get lost as anything else inside. What should go on a piece of paper is your destination information if your permanent contact information won't reach you at your destination - if you won't be able to check email, for instance, or if you'll be out of cell phone range.)
It isn't only the USA that opens cases, anyway. That's worldwide.
Speed comes from what you put behind you. - Judi Ketteler