Much more complicated than that. The brake light actuator takes input from multiple sources including an accelerometer, speed, power or rate of regen and more. This has been discussed at length wrt the Roadster and the software was tweaked a few times.
This probably needs a new thread, so mods, feel free to move this...
I agree that having the brake lights come on as soon as you lift the accelerator pedal is weird. Nothing (well, almost nothing) annoys me more than people dabbing the brakes at speed when it's completely unnecessary.
Back in the day, I used to have an Astra GTE (manual/stick) that had an LCD dash (ooooh!), and a few lights to check various functions, including brake lights. The warning light would stay lit until the first brake, where it would acknowledge the lights were working, and extinguish. My big challenge every day was to get to the office without braking (~10 miles, light traffic), and I used to manage it every now and then. My point being, the regen is generally 'sold' as being a bit like a manual, when if you drop a gear, the car will slow. In that case, the brake lights don't come on, and people behind don't get stressed by it. I'm sure the logic for the Model S is complicated, and I've no idea how a Volt (for example) does it, but the current set up seems flawed. If I know that I 'brake' as soon as I lift off, I'll be overly-conscious of managing the accelerator pedal, which is a little distracting.
Of course, I say all of this based on this thread and a couple of test drives, so actual owners may have a more relevant point of view.