The theory is that backlight bleed can spread when the LED panel heats up, which I can't prove or disprove based on that notion. From my understanding and experience with LED panels, I have not seen backlight bleed expand beyond its current state.
The reason you don't see every LED device, iPad or TV being returned because of back light bleed is not everyone has eyes that are tuned to observe what back light bleed is and how to recognize it. Backlight bleed is Common, but some cases are hardly noticeable and others are extreme. Again, it all depends on how the panels are assembled from the manufacture.
The reason you tend to notice it, is because you frequent tech forms and you have an idea of what it is, and you work in the store where items are returned every day for defects. But if you take the average individual, who purchases a TV, may not recognize the backlight bleed just based off not knowing what it is.
Also, it has nothing to do with something that you're doing to the device to cause backlight bleed. It's the LED panel and glass having a gap that is allowing light pass through. User settings on TV's can make backlight bleed more noticeable or less depending on what you have it set for.