Basically, in the US the UL is the standard which one follows (CE if
you want to go elsewhere). Failing to meet the UL/CE standards
generally has force of law in the US due to the ever diligent lawyers.
I feel these standard, or at least the 'CE mark' is part of the problem,
and here's why....
It would apperar there are either loopholes in the standard, or plenty of
cheap electrical devices (including wall-warts) that have invalid CE
marks on them, in that I've seem enough devices that I don't consider
safe. The problem is that if the wall-wart carries the CE mark, then as
you said, the certification for the whole product becomes a lot easier,
and if there are any problems later, a large part of the defence is 'But
the wall wart met the appropriate CE standards'.
But when manufacturers were entirely responsible for what their devices
did, they made darn sure they were safe. Said manufacturers did not want
to end up paying out large amounts of damages. SO th products really
_were_ safe.
I offer as an example the PSU brick for the Philips G7000 video game.
It's a bit like awall-wart, except it has a mains input cable that you
wire to a suitale plug. But inside that brick (which is nicely screwed
together), there are no fewer than 3 protective devices. A mains-side
fuse, a secondary fuse, and a thermal fuse (on the mains side).
-tony