I'm sure they're not extractable at scale, otherwise we'd have heard about it, but they're almost certainly extractable on a one-off basis if you have a big enough budget (one that likely outweighs whatever you'd stand to gain from it). It's just not the low hanging fruit of the system as a whole.
I also expect it not to be the low-hanging fruit of these cameras, but it is still one possible attack vector.
It still serves a legal purpose, as long as it's difficult enough. If you need to physically modify the device to access the keys, then bringing the unmodified device to court should be enough to verify legitimacy of the signature.
I'm sure they can make it hard, with secure silicon etc., but they'll never make it impossible and that somewhat defeats the purpose.