I’m a little more sinister...I believe the real reason is they consider them to be a waste of energy thus not green!!!The UL designation is but a ruse to get a regulation passed that can be interpreted to mean what they want!!
I think it is more about non compliant imports from certain countries. I know the older string lights are scary hot if left on for too long.
Maybe I should find some of those lights. Green compliance will be enforced at the source and I may need to huddle around those lights for warmth.