That is the Eighth Amendment to the US Constitution.
Here is how Justice Ginsburg interpreted it: “The prohibition embodied in the Excessive Fines Clause carries forward protections found in sources from Magna Carta to the English Bill of Rights to state constitutions from the colonial era to the present day,” Ginsburg stated. “Protection against excessive fines has been a constant shield throughout Anglo-American history for good reason: Such fines undermine other liberties.”
Even those of us who wish Trump would simply disapear feel that a $400 million+ dollar fine in a case that did not involve any damages to anyone qualifies as cruel and far more than unusual AND sets a precedent that will severely inhibit economic activity.
Link: https://en.wikipedia.org/wiki/Eighth_Amendment_to_the_United_States_Constitution