For those who haven't driven cars with regen facility, it's worth noting the following, for my car at least. The regen takes effect when 'coasting', as it were, in addition to deliberately slowing. Or even if merely lifting off the throttle. I can see what my car is doing with regen. Often it will be when going down an inclune, or even on a motorway. There is a negligible decrease in speed. If brake lights came on as soon as regen was activated in all cases then there would be a major problem. Lights coming on and off, those behind braking in response causing bunching, over reaction, etc.
I'd agree that the lights should come on if there is a significant braking effect from the regen system. This is what most well designed EVs do.