Very few things in law are truly reducible to some hard, bright-line rule. There are conflicting interests all worthy of consideration and that therefore defy a single objective rule. So even in the presence of extensive precedent, there is usually a grey area requiring case-by-case judgment based on intuitions of fairness and equity.
Say you own an apartment building with a view of the ocean and I own the plot directly between it and the ocean. I threaten to build a wall that blocks your building's view of the ocean unless you buy my plot for 10x more than its market worth. Should that be legal? Say you succumb to my demand. Should you be able to then sue me and recover the excessive payment? I think many would have the intuition that my threat should not be legal and any resulting contract not valid, since it was extracted by duress.
But how about Firefox selling its search bar to Google for billions of dollars, carrying the implicit threat to use a different default provider should Google not pay. That seems more fair than the previous example, but why?
And how do you formulate that difference into a hard rule, without relying on human beings' (i.e. judge and jury) intuitions of fairness and equity?
Here is what the Restatement of Contracts has to say:
(1) A threat is improper if
(a) what is threatened is a crime or a tort, or the
threat itself would be a crime or a tort if it
resulted in obtaining property,
(b) what is threatened is a criminal prosecution,
(c) what is threatened is the use of civil process
and the threat is made in bad faith, or
(d) the threat is a breach of the duty of good faith
and fair dealing under a contract with the
recipient.
(2) A threat is improper if the resulting exchange is
not on fair terms, and
(a) the threatened act would harm the recipient and
would not significantly benefit the party making
the threat,
(b) the effectiveness of the threat in inducing the
manifestation of assent is significantly
increased by prior unfair dealing by the party
making the threat, or
(c) what is threatened is otherwise a use of power
for illegitimate ends.
Look at all of the subjective terminology: "bad faith", "good faith", "fair dealing", "fair terms", "significantly".
And look at (2)(c), "otherwise a use of power for illegitimate ends". What does that even mean? It's basically a surrender, acknowledging that it is impossible to formulate a rule ex ante that totally captures our notions of fairness as they should be applied in every possible situation.