<font face="Verdana, Arial, Helvetica, sans-serif" size="2">US carriers, on the other hand, have decided (or maybe it is an FAA regulation, not sure) that it is safer for parents to simply hold small children on the parents' lap with no belt.</font>
In 1996 the FAA banned anything other than approved car seats for takeoff, landing and taxi. There are private companies that sell the vests similar to what you describe European airlines as offering and supposedly you can use those during the cruise portion of the flight. I saw one of these on the Today Show recently. I think it was this one.
http://www.babybair.com/product.htm