> We can't just say that because it's legal then it's ok.

Absolutely. I say this all the time.

> Prosecutors can decline to prosecute.

But prosecutors can also choose to prosecute. When there's that much give in the legal code (giving a ride falls between 'no charge' and the death penalty), there's a lot of room for abuse and corruption. It might be worth giving up the ability to execute people for guarding (terrorist) warehouses for the assurance that we are governed by laws and not by (fickle, prejudiced, and corruptible) men.