Jury service is one of the few things required of a citizen by the mere act of existing. States require car insurance and driver’s licenses, but only if you want to drive a car. Many municipalities require you to keep your home up to certain standards, but if you don’t like that, you don’t have to live there.
Even paying income taxes is only required of you if you earn income—admittedly, not something most of us can afford to opt out of, but it is an option nonetheless. Much of the controversy around Obamacare stemmed from the requirement that all citizens, regardless of any action they have or have not taken, must purchase health insurance.
Americans don’t like being told what to do, and we have long held an essential part of our liberty to be the right to say “no.” All governments do things some of their citizens object to, but when those citizens are not directly involved, they often do nothing to stop it. They may vote against the proponents of such a policy, but they do not start a revolution over every disagreement. They ignore it, and implicitly consent.
When that same government makes the average citizen an instrument of that policy, however, the citizen might think harder about it, and remember that consent may be withdrawn. When a criminal law is unjust, or when the punishment for violating it too harsh, the jury cannot be expected to become a part of that injustice.