Participation in the system is voluntary on both sides, as the Sixth Amendment guarantees the right to a trial by jury.
One minor technical correction: it's the
seventh amendment to the U.S. Constitution that guarantees a trial by jury in civil cases:
In Suits at common law, where the value in controversy shall exceed twenty dollars, the right of trial by jury shall be preserved, and no fact tried by a jury, shall be otherwise re-examined in any Court of the United States, than according to the rules of the common law.
The seventh amendment is somewhat unusual in that in only applies to federal litigation. However, as far as I know most U.S. states require jury trials in civil cases within their jurisdiction.
The
sixth amendment requires jury trials for criminal prosecutions, and applies to the state as well as the federal courts.