Originally Posted by
GUWonder
The US has de jure and de facto demanded that common carrier airlines require passenger passports, NEXUS card, or passport replacing docs as a condition to transport recognized US citizens (and others) out of the US. This was done by the US under a US move referred to as the WHTI. If US carriers ignore WHTI when flying passengers out of the US, the carriers are subject to a penalty.
In other words, these restrictions are not merely a product of destination country rules, and this is something that is still evident with regard to air travel by US citizens who arrive in Canada and use a US state driving license and US state birth certificate to seek admission into Canada at a Canadian airport of entry.
Fair point, I had forgotten about WHTI. Still, that's only a partial limitation. It doesn't extend to trips outside of North/South America, the Caribbean, or Bermuda. You can still leave the country without a passport if you're flying to London or Singapore, for example. BA or SQ won't let you on the plane, most likely (due to the admission issue), but if you were flying private, you could probably pull it off.