That's because you don't understand sin then. I'm sure Adam and Eve didn't think eating a fruit would cause so many problems but as we can see even a 'tiny' sin ended up leading to the murder of Abel by his own brother. Sin is sin and although you may think a 'tiny' sin is not something worth eternal punishment you have to take into consideration that you have been corrupted and once that seed of sin is planted it only gets worse. It goes a lot deeper than what you think. When you have defied God you have in effect defied what is good for what is evil. You have in effect already killed yourself at the first 'tiny' sin because we know the horrid things sin leads to and that God said sin leads to death. What is stopping a sinner from eventually getting worse if they can't even follow God who is good on a tiny thing? I hope you understand what I'm trying to say.
You might be surprised that I agree with you 100%. The only problem I have is in the way you are trying to frame this situation. It is not a courtroom problem, this is what they tried to propose in the RCC and it eventually led them to define an upper stage of hell called limbo for the fate of unbaptized infants and moral pagans. In recent times they have largely abandoned that theory because of aborted Catholic children.
See, in a court room, if you steal a pen, the "just" punishment would be to return the pen and pay some fine or penalty proportional to the value of the pen, which would not be very much. No reasonable person is going to accept that a fair punishment for the crime is life in prison. In atheist China, the government routinely executes individuals for relatively minor crimes: drug dealing, thievery etc. Most Christians look at that system as cruel because of the rule of law which dictates the punishment must fit the crime. This dictate comes from the Bible's provisions "An eye for an eye" within the Mosaic law. What the Mosaic law does not say is, "Two eyes, an arm and a leg for an eye." Proper justice promotes equitable punishment proportional to a crime.
What you are describing, is what I call the medical perspective. The punishment for choosing not to take your medication is the full force of whatever affliction you are dealing with. This punishment, however, is not so much "punitive" as it is "consequential". Its not so much that your actions are engendering the wrath of a moody God, its that you are deliberately turning yourself against His will and therefore experiencing His love as wrath.
This "medical" explanation is how we ultimately are able to show people how sin merits eternal punishment. Its not really God who is even punishing the person, the person is punishing themselves by continuing in arrogance and self-love when the medicine (the New Covenant) is put right within their reach, if they would only humble themselves and take it.
While stealing one penny in the judicial system is indeed "no big deal", contracting just a single E Bola virus is all it takes to give you the disease.