(governments) aren't obligated to enforce every law on the book, they are usually content just HAVING the law on the book
This is simply evil and, in my personal opinion, flagrantly immoral. In my book it completely undermines a government's legitimacy.
The first country to make "the state failed to enforce it uniformly" a valid defense against charges of breaking a law will win at least a few wealthy immigrants happy to pay lots of tax. Without "failure to enforce" as a defense, the legislature has no incentive to limit the scope of the law.
If the public prosecutor's budget exceeds the public defender's budget, the court is illegitimate.
It's probably less immoral then hiring an army of police to enforce both laws against murder and spitting on the sidewalk at the same time. They enforce what they think is worth while enforcing. Things that are genuinely dangerous for society and things that have nice big fat monetary penalties that the legal system gets to keep, tend to be high on the list of enforced laws.
Just how it goes. Immoral or not.