I'm curious about this whole PWM thing. I'm an electronics engineer. Now if I want to control and LED of some power I would not use raw PWM. It's crude, not efficient and prone to generate RFI which will make it doubtful that the device would pass an EMC test required by CE or UKCA certifications. I guess some manufacturers may be using it to cheap out on a few components but these must be the very cheapest of devices.
And yes any PWM used will be at such a high frequency that you cannot see it directly, what you typically see is heterodyning between the PWM frequency and the refresh rate (frequency) of the camera. You get a similar effect if you look at a CRT screen through a fan in front of it.
Yes PWM is involved in controlling LED's, it will be at least 100kHz and up to 1MHz and the LED's do not turn on and off. LED's are controlled with an LED driver which is basically a switch mode power supply, this is the same type of power supply that powers your whole computer not to mention every one of the many voltage conversion stages inside the computer. The difference is that while a power supply is regulating around the voltage output the LED driver is regulating around the current output (with a voltage ceiling) as LED's do not respond well the being connected across a voltage source, they go bang! But it's the same power converter and it will be aiming to maintain a steady flow of current. So you won't see all this flickering anyway. Well there will be something like 10% "ripple" at 100+ kHz.