on using seat belts

I don't understand the reluctance of manufacturers? users? service providers? of public transportation in the US to install seat belts.  Obviously airlines understand the necessity on airplanes.  Personally, in years past, seat belts have saved both my son and me from serious injury in car mishaps. When I traveled in England four years ago we took a couple of bus tours and the driver wouldn't start moving until everyone was buckled up.  It was the law.  Seat belts were on all of the buses we used on our Croatia trip. So why not here in the US?  The recent train wreck outside of Philadelphia got me wondering about this again.  This was a violent crash, killing eight people, and I'm not sure how many could have been saved with seat belts but I heard at least one commentator theorizing that at least some deaths and injuries could have been prevented if the train had been equipped with seat belts.  So why not have them available, even if you don't want to enforce their use?  OK, I'll get off my soap box now (as soon as I unfasten my seat belt).