Exactly, fungibility is a problem, not a feature. If the law were not fungible and the police were obligated to charge any violations, and the DA was obligated

Exactly, fungibility is a problem, not a feature. If the law were not fungible and the police were obligated to charge any violations, and the DA was obligated to prosecute any charges, then the law becomes a nuisance to everyone and we'd see a lot more citizen involvement in reducing that nuisance by more strictly limiting state power. Reducing that nuisance by making the law fungible instead means those in power people get to be selective and make exceptions for friendlies while oppressing their opponents.