Are you sure, I don't think this is true. All christmas lights I have seen for 5+ years have a little switching regulator in the power supply box. A lot of strings (Especially cheaper ones) do now have those hideous plug, PSU and multi-function controller things, and only two wires leaving the plug to the lights. The lights are wired with each LED being the opposite polarity to the previous. They do the effects with PWM, but when you have both sets on, there is a slight flicker as the polarity has to be swapped very fast to keep them on.