You might think that there would have been more important things for NHTSA to worry about than the safety of World War II era amphibious vehicles with no particular commercial or military value. However, a marketing genius decided to convert them into tourist attractions in coastal cities such as Seattle. One of them lost an axle on the Aurora bridge and swerved into an oncoming tour bus, killing and maiming passengers in both vehicles. Another one sank in a thunderstorm, killing all aboard (I believe.) While you could argue that the passengers on the vehicle should have known the risks of riding a discarded military assault craft, certainly the Japanese tourists in the properly modern and licensed tour bus had the right to assume that all of the other vehicles on the road were as safe as modern practice can make them.
If there is an unsafe old aircraft out there, somebody is going to find a reason to fly it; maybe they got it for free so in the short term the fuel costs are immaterial. In my opinion, a few man weeks of engineering time is really a small price to pay to ensure that at least the obvious faults are either fixed or the airframe is grounded. Owners of old commercial buildings have have to spend millions to retrofit them to survive earthquakes; a much grumbled about cost but when an earthquake causes mass casualties in other countries the first thing we point to is "lax building codes."