Illegal isn't binary - just look at speed limits. Everyone speeds at least a little, and FSD/AP had to be allowed to speed to be safe
Illegal is binary for most driving laws (especially 'do what this sign says' rules), but some things fall into an ambiguous category of illegal-but-rarely-enforced. The problem is that computers don't do well with ambiguous rules. I strongly suspect that when most cars are using FSD the rule of being allow to drive a little over the speed limit will be removed, and cars will have to stick to the limits. Hopefully the limits will be raised.
Whereas, a rolling stop, that's not legal (as far as I know) anywhere.
EDIT: I am incorrect with the above statement. I apparently live in one of 4 states where you can exceed the speed limit by up to 10mph while passing.
For sure in Colorado it's not legal to exceed the speed limit to pass. But it is illegal to drive below the speed limit while in the passing lane on a highway with a speed limit 65 mph or higher. There's exceptions for safety and congestion, but otherwise the left lane is considered a passing lane. If you're not passing, you're not supposed to be in that lane.
Of course these are the laws, not the practice, which is what I think GP was trying to say.
Of all of the places I've been Chicago was probably the worst at speeding, especially in the dead of night when the roads are "too" open. Recently they got a bit stricter with the speed cameras though https://www.illinoispolicy.org/chicagos-speed-cameras-ticket...
And again, I don't think the car should speed by default, but a car going 2-3mph over the posted limit should be acceptable rather than an error state because the world is not black and white.
If you can determine in real-time whether the maneuver your about to perform or the speed you're going is safe then why even have speed limits? The speed limit for highways is still 65 whether it's a a bone dry, pitch dark, pouring rain, or completely iced over. "Any speed under 65" can't possibly be a safe speed for all these conditions while allowing for the highest safe speeds possible in ideal, or even average, conditions. And this doesn't even being to take into account the huge vehicle variance and tire ware. The safe operating speeds for a top-heavy Honda Fit with narrow tires vs a low-to-the-ground wide-tired Corvette are going to be wildly different.
And then you have to deal with other drivers. If traffic is going 75 you're gonna have a hell of a time merging capped at 65. And in an ideal world nobody would pass on the right making it possible to get off the highway without increasing speed but real life hits hard.
For the same reason driving below average speed is dangerous. If you drive 10km/h slower than everyone else you are a problem, even if everyone else is driving at or slightly above the speed limit (very common in Germany).
I would strongly discourage you from trying that line of argument with the cops when you're pulled over for speeding. I'm certain that a court of law with decent standards does not interpret speeding "at least a little" as anything except a binary.
A thing is not legal just because law enforcement lets you get away with it, but I don't think this is a good example. It's silly to have this conversation without at least acknowledging the way driver behavior expands into "grey areas" and gaps in enforcement, and I don't think it's trivially obvious that "self driving" cars should rigidly follow the letter of the law even if that means they'll be the only cars on the road doing so.