“But if they don't like it, their only choice is to flee Japan. “
No its not. They can become outlaws. Do not some martial arts weapons descend from farm implements because of this? The peasants took a dim view of being denied arms so went outside the law and turned their farm implements into weapons.
They can also force change and “…institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.
“That's not a "contract"; the peasants in this case most certainly never agreed to anything.”
One could argue they “signed” their agreement by not fleeing, becoming outlaws or revolting.
“So this obviously unjust institution fits your idea of a ‘social contract’…”
Unfounded attribution—at least I don’t recall saying any such thing.
“…and the "social contract" in the US has similarly bad implications--for example, that policemen can break into a house, shoot the occupants, explain that they meant to raid the "crack house" next door, and get away with murder. I can't escape this injustice without fleeing the country.”
Looks like an establishment of imitation and approximation probably for the purpose of having an easy target.
You mean, they can opt out of the contract they never opted into, by the trivial expedient of having a price set on their heads and spending the rest of their natural lives on the lam? Oh, silly me!
One could argue they “signed” their agreement by not fleeing, becoming outlaws or revolting.
That's precisely your (and tpaine's) argument, yes. You are saying that I can come to your house and declare myself your master, and it's a legally binding contract given that you didn't flee? Um, what gave me the right to go make such a declaration in the first place? And since fleeing is incompatible with exercising your property rights, how did I get the authority to tell you that you either (1) accept my contract, or (2) forfeit your property rights? Do you actually know the word "duress"?