Sure, the same way that Christmas-tree light strings
are "safe".
Were it not for tradition, I cannot imagine that any underwriting or
certifying agency would approve of line current-powered strings of
low-voltage series-connected lamps on lightweight insulated #20 wire
I beleive that Christmas lights are one of the few (if not the only)
single-insulated non-earthed device that can be sold in the UK. Althoguh
a lot of the better-grade light sets sold over here are either
low-voltage bulbs or LEDs effectively in parallel running of a 12V or 24V
wall-wart (for all I hate the latter ,they are safer than the older type
of series-string lights)
I also wonder how the light bulb sockets (both the BC (Baynoet Cap) ones
used in the UK and the ES ones used elsewhere are approved. With the bulb
removed (which can be done without the use of a tool), there are exposed
livel parts. (Yes, I know the better makes of BC holder contain automatic
switches to break the connections when the bulb is removed, but an awful
lot of holders without said switches are around).
-tony